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Detached eclipsing binaries (DEBs) allow for the possibility of precise characterization of its stellar components. They offer a unique opportunity to derive their physical parameters in a near-model-independent way for a number of systems consisting of late-type giant stars. Here we aim to expand the sample of low-metallicity late-type giant stars with precise parameters determined. We aim to determine the fundamental parameters like the mass, radius, or effective temperature for three long-period late-type eclipsing binaries from the Large Magellanic Cloud: OGLE-LMC-ECL-25304, OGLE-LMC-ECL-28283, and OGLE-IV LMC554.19.81. Subsequently we aim to determine the evolutionary stages of the systems. We fit the light curves from the OGLE project and radial velocity curves from high resolution spectrographs using the Wilson-Devinney code. The spectral analysis was performed with the GSSP code and resulted in the determination of atmospheric parameters such as effective temperatures and metallicities. We used isochrones provided by the MIST models based on the MESA code to derive evolutionary status of the stars. We present the first analysis of three DEBs composed of similar He-burning late-type stars passing through the blue loop. Estimated masses for OGLE-LMC-ECL-29293 (G4III + G4III) are $M_1=2.898\pm0.031$ and $M_2=3.153\pm0.038$ $M_\odot$, stellar radii are $R_1=19.43\pm0.31$ and $R_2=19.30\pm0.31$ $R_\odot$. OGLE-LMC-ECL-25304 (G4III + G5III) has stellar masses of $M_1=3.267\pm0.028$ and $M_2=3.229\pm0.029$ $M_\odot$, radii of $R_1=23.62\pm0.42$ and $R_2=25.10\pm0.43$ $R_\odot$. OGLE-IV LMC554.19.81 (G2III + G2III) have masses of $M_1=3.165\pm0.020$ and $M_2=3.184\pm0.020$ $M_\odot$, radii of $R_1=18.86\pm0.26$ and $R_2=19.64\pm0.26$ $R_\odot$. All masses were determined with a precision better than 2\% and radii better than 1.5\%. The ages of the stars are in the range of 270-341 Myr., Comment: 15 pages, 15 figures. Accepted for publication in A&A

Separating C6 cyclic hydrocarbons, specifically benzene and cyclohexane, presents a significant industrial challenge due to their similar physicochemical properties. We conducted Monte Carlo simulations in the Grand-Canonical ensemble to acquire adsorption properties and separation performance data for benzene and cyclohexane in three metal-organic frameworks featuring coordinatively unsaturated metal sites (Ni-MOF-74, Ni-ClBBTA, and Ni-ClBTDD). The separation performance of these MOFs was analyzed and compared with literature data for adsorbents of different natures, demonstrating superior performance. Additionally, we explored the molecular origins of this effective separation, examining the pore-filling mechanism, interaction of guest molecules with metal centers, and mutual interactions of each adsorbate. Our results highlight that the selected adsorbents, with remarkable loading capacity, can efficiently separate both compounds in a mixture with exceptional effectiveness.

The building climate industry and its influence on energy consumption have consequences on the environment due to the emission of greenhouse gasses. Improving the efficiency of this sector is essential to reduce the effect on climate change. In recent years, the interest in porous materials in applications such as heat pumps has increased for their promising potential. To assess the performance of adsorption heat pumps and cooling systems, here we discuss a multistep approach based on the processing of adsorption data combined with a thermodynamic model. The process provides properties of interest, such as the coefficient of performance, the working capacity, the specific heat or cooling effect, or the released heat upon adsorption and desorption cycles, and it also has the advantage of identifying the optimal conditions for each adsorbent-fluid pair. To test this method, we select several metal-organic frameworks that differ in topology, chemical composition, and pore size, which we validate with available experiments. Adsorption equilibrium curves were calculated using molecular simulations to describe the adsorption mechanisms of methanol and ethanol as working fluids in the selected adsorbents. Then, using a thermodynamic model we calculate the energetic properties combined with iterative algorithms that simultaneously vary all the required working conditions. We discuss the strong influence of operating temperatures on the performance of heat pump devices. Our findings point to the highly hydrophobic metal azolate framework MAF-6 as a very good candidate for heating and cooling applications for its high working capacity and excellent energy efficiency.

The book consists of a number of short articles that present achievements of the Araucaria members, collaborators, and friends, in various aspects of distance determinations and related topics. It celebrates the 20-year anniversary of the Araucaria Project, acknowledges the people who worked for its success, and popularises our methods and results among broader readership. This book is a part of a project that has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 695099., Comment: 114 pages, book published in 2021 on behalf of the Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences, to celebrate 20 years of the Arauria Project

Our goal is to determine, with high accuracy, the physical and orbital parameters of two double-lined eclipsing binary systems, where the components are two giant stars. We also aim to study the evolutionary status of the binaries, to derive the distances towards them by using a surface brightness-colour relation, and to compare these measurements with the measurements presented by the Gaia mission. In order to measure the physical and orbital parameters of the systems, we analysed the light curves and radial-velocity curves with the Wilson-Devinney code. We used V band and I-band photometry from the OGLE catalogue and near-infrared photometry obtained with the New Technology Telescope (NTT) equipped with the SOFI instrument. The spectroscopic data were collected with the HARPS spectrograph mounted at the ESO 3.6m telescope and the MIKE spectrograph mounted at the 6.5m Clay telescope. We present the first analysis of this kind for two evolved eclipsing binary systems from the OGLE catalogue: OGLE-BLG-ECL-305487 and OGLE-BLG-ECL-116218. The masses of the components of OGLE-BLG-ECL-305487 are $M_1$ = 1.059 $\pm$ 0.019 and $M_2$ = 0.991 $\pm$ 0.018 $M_\odot$, and the radii are $R_1$ = 19.27 $\pm$ 0.28 and $R_2$ = 29.99 $\pm$ 0.24 R$_\odot$. For OGLE-BLG-ECL-116218, the masses are $M_1$= 0.969 $\pm$ 0.012 and $M_2$= 0.983 $\pm$ 0.012 $M_\odot$, while the radii are $R_1$= 16.73 $\pm$ 0.28 and $R_2$= 22.06 $\pm$ 0.26 $R_\odot$. The evolutionary status of the systems is discussed based on the PARSEC and MIST isochrones. The ages of the systems were established to be between 7.3-10.9 Gyr for OGLE-BLG-ECL-305487 and around 10 Gyr for OGLE-BLG-ECL-116218. We also measured the distances to the binaries. For OGLE-BLG-ECL-305487, $d$= 7.80 $\pm$ 0.18 (stat.) $\pm$ 0.19 (syst.) kpc and for OGLE-BLG-ECL-116218, $d$= 7.57 $\pm$ 0.28 (stat.) $\pm$ 0.19 (syst.) kpc., Comment: 12 pages, 5 figures, accepted in A&A

Surface brightness -- colour relations (SBCRs) are very useful tools for predicting the angular diameters of stars. They offer the possibility to calculate very precise spectrophotometric distances by the eclipsing binary method or the Baade-Wesselink method. Double-lined Detached Eclipsing Binary stars (SB2 DEBs) with precisely known trigonometric parallaxes allow for a calibration of SBCRs with unprecedented precision. In order to improve such calibrations, it is important to enlarge the calibration sample of suitable eclipsing binaries with very precisely determined physical parameters. We carefully chose a sample of ten SB2 DEBs in the solar neighbourhood which contain inactive main-sequence components. The components have spectral types from early A to early K. All systems have high-precision parallaxes from the Gaia mission. We analysed high precision ground- and space-based photometry simultaneously with the radial velocity curves derived from HARPS spectra. We used spectral disentangling to obtain the individual spectra of the components and used these to derive precise atmospheric parameters and chemical abundances. For almost all components, we derived precise surface temperatures and metallicities. We derived absolute dimensions for 20 stars with an average precision of 0.2% and 0.5% for masses and radii, respectively. Three systems show slow apsidal motion. One system, HD 32129, is most likely a triple system with a much fainter K6V companion. Also three systems contain metallic-line components and show strong enhancements of barium and ittrium. The components of all systems compare well to the SBCR derived before from the detached eclipsing binary stars. With a possible exception of HD 32129, they can be used to calibrate SBCRs with a precision better than 1% with available Gaia DR3 parallaxes., Comment: Accepted in Astronomy & Astrophysics, 18 pages

We present and analyze optical photometry and high resolution SALT spectra of the symbiotic recurrent nova V3890 Sgr at quiescence. The orbital period, P=747.6 days has been derived from both photometric and spectroscopic data. Our double-line spectroscopic orbits indicate that the mass ratio is q=M_g/M_WD=0.78+/-0.05, and that the component masses are M_WD=1.35+/-0.13 Msun, and M_g=1.05+/-0.11 Msun. The orbit inclination is approximately 67-69 degr. The red giant is filling (or nearly filling) its Roche lobe, and the distance set by its Roche lobe radius, d=9 kpc, is consistent with that resulting from the giant pulsation period. The outburst magnitude of V3890 Sgr is then very similar to those of RNe in the Large Magellanic Cloud. V3890 Sgr shows remarkable photometric and spectroscopic activity between the nova eruptions with timescales similar to those observed in the symbiotic recurrent novae T CrB and RS Oph and Z And-type symbiotic systems. The active source has a double-temperature structure which we have associated with the presence of an accretion disc. The activity would be then caused by changes in the accretion rate. We also provide evidence that V3890 Sgr contains a CO WD accreting at a high, a few 1e-8 - 1e-7 Msun/yr, rate. The WD is growing in mass, and should give rise to a Type Ia supernova within about 1,000,000 yrs - the expected lifetime of the red giant., Comment: 12 pages, 9 figures; accepted for publication in MNRAS

The surface brightness -- colour relation (SBCR) is a basic tool in establishing precise and accurate distances within the Local Group. Detached eclipsing binary stars with accurately determined radii and trigonometric parallaxes allow for a calibration of the SBCRs with unprecedented accuracy. We analysed four nearby eclipsing binary stars containing late F-type main sequence components: AL Ari, AL Dor, FM Leo and BN Scl. We determined very precise spectroscopic orbits and combined them with high precision ground- and space-based photometry. We derived the astrophysical parameters of their components with mean errors of 0.1% for mass and 0.4% for radius. We combined those four systems with another 24 nearby eclipsing binaries with accurately known radii from the literature for which $Gaia$ EDR3 parallaxes are available, in order to derive the SBCRs. The resulting SBCRs cover stellar spectral types from B9 V to G7 V. For calibrations we used Johnson optical $B$ and $V$, $Gaia$ $G_{\rm BP}$ and $G$ and 2MASS $JHK$ bands. The most precise relations are calibrated using the infrared $K$ band and allow to predict angular diameters of A-, F-, and G-type dwarf and subgiant stars with a precision of 1%., Comment: Accepted for publication in Astronomy & Astrophysics, 17 pages

Besides the astrometric mission of the Gaia satellite, its repeated and high-precision measurements serve also as an all-sky photometric transient survey. The sudden brightenings of the sources are published as Gaia Photometric Science Alerts and are made publicly available allowing the community to photometrically and spectroscopically follow-up the object. The goal of this paper was to analyze the nature and derive the basic parameters of Gaia18aen, transient detected at the beginning of 2018. It coincides with the position of the emission line star WRAY 15-136. The brightening was classified as a "nova?" on the basis of subsequent spectroscopic observation. We have analyzed two spectra of Gaia18aen and collected the available photometry of the object covering the brightenings in 2018 and also the preceding and following periods of quiescence. Based on this observational data, we have derived the parameters of Gaia18aen and discussed the nature of the object. Gaia18aen is the first symbiotic star discovered by the Gaia satellite. The system is an S-type symbiotic star and consists of an M giant of a slightly super-solar metallicity, with Teff ~3500 K, a radius of ~230 R$\odot$, and a high luminosity L ~7400 L$\odot$. The hot component is a hot white dwarf. We tentatively determined the orbital period of the system ~487 days. The main outburst of Gaia18aen in 2018 was accompanied by a decrease in the temperature of the hot component. The first phase of the outburst was characterized by the high luminosity L ~27000 L$\odot$, which remained constant for about three weeks after the optical maximum, later followed by the gradual decline of luminosity and increase of temperature. Several re-brightenings have been detected on the timescales of hundreds of days., Comment: accepted for publication in Astronomy & Astrophysics; 10 pages, 10 figures, 5 tables

Context. EE Cep is one of few eclipsing binary systems with a dark, dusty disk around an invisible object similar to {\epsilon} Aur. The system is characterized by grey and asymmetric eclipses every 5.6 yr, with a significant variation in their photometric depth, ranging from ~ 0 m .5 to ~ 2 m .0. Aims. The main aim of the observational campaign of the EE Cep eclipse in 2014 was to test the model of disk precession (Galan et al. 2012). We expected that this eclipse would be one of the deepest with a depth of ~ 2 m .0. Methods. We collected multicolor observations from almost 30 instruments located in Europe and North America. This photometric data covers 243 nights during and around the eclipse. We also analyse the low- and high-resolution spectra from several instruments. Results. The eclipse was shallow with a depth of 0 m .71 in V-band. The multicolor photometry illustrates small color changes during the eclipse with a total amplitude of order ~ +0 m . 15 in B-I color index. The linear ephemeris for this system is updated by including new times of minima, measured from the three most recent eclipses at epochs E = 9, 10 and 11. New spectroscopic observations were acquired, covering orbital phases around the eclipse, which were not observed in the past and increased the data sample, filling some gaps and giving a better insight into the evolution of the H {\alpha} and NaI spectral line profiles during the primary eclipse. Conclusions. The eclipse of EE Cep in 2014 was shallower than expected 0 m .71 instead of ~ 2 m . 0. This means that our model of disk precession needs revision., Comment: 10 pages, 13 figures, 29 tables in appendix, submited to Astronomy & Astrophysics

Hen 3-160 is reported in Belczynski et al.'s (2000) catalog as a symbiotic binary system with M7 giant donor. Using $V$- and $I$-band photometry collected over 20 years we have found that the giant is a Mira variable pulsating with 242.5-day period. The period-luminosity relation locates Hen 3-160 at the distance of about 9.4 kpc, and its Galactic coordinates ($l=267.7^{\circ}$, $b=-7.9^{\circ}$) place it $\sim$1.3 kpc above the disc. This position combined with relatively high proper motions (pm$_{\rm{RA}}=-1.5$ mas yr$^{-1}$, pm$_{\rm{DEC}}=+2.9$ mas yr$^{-1}$, Gaia DR2) indicates that Hen 3-160 has to be a Galactic extended thick-disc object. Our red optical and infrared spectra show the presence of ZrO and YO molecular bands that appear relatively strong compared to the TiO bands. Here we propose that the giant in this system is intrinsic S star, enriched in products of slow neutron capture processes occurring in its interior during an AGB phase which would make Hen 3-160 the first symbiotic system with Mira variable S star., Comment: accepted for publication at Acta Astronomica

V934 Her = 4U1700+24 is an M giant-neutron star (NS) X-ray symbiotic (SyXB) system. Employing optical and infrared radial velocities spanning 29 years combined with the extensive velocities in the literature, we compute the spectroscopic orbit of the M giant in that system. We determine an orbital period of 4391 days or 12.0 yr, the longest for any SyXB, and far longer than the 404 day orbit commonly cited for this system in the literature. In addition to the 12.0 yr orbital period we find a shorter period of 420 days, similar to the one previously found. Instead of orbital motion, we attribute this much shorter period to long secondary pulsation of the M3 III SRb variable. Our new orbit supports earlier work that concluded that the orbit is seen nearly pole on, which is why X-ray pulsations associated with the NS have not been detected. We estimate an orbital inclination of 11.3$^\circ$ $\pm$ 0.4$^\circ$. Arguments are made that this low inclination supports a pulsation origin for the 420 day long secondary period. We also measure CNO and Fe peak abundances of the M giant and find it to be slightly metal poor compared to the Sun with no trace of the NS forming SN event. Basic properties of the M giant and NS are derived. We discuss the possible evolutionary paths that this system has taken to get to its current state., Comment: accepted for publication in ApJ

We report finding high-velocity components in the H{\alpha} emission wings of St 2-22 spectra, obtained in 2005. This discovery have encouraged us to start the present study, aiming to show that this little studied object is a jet producing symbiotic system. We used high-resolution optical and low-resolution near infrared spectra, as well as available optical and infrared photometry, to evaluate some of the physical parameters of the St 2-22 components and the characteristics of the jets. The evaluated parameters of the components confirmed that St 2-22 is a S-type symbiotic star. Our results demonstrate that an unnoticed outburst of St 2-22, similar to those in classical symbiotics, occurred in the first half of 2005. During the outburst, collimated, bipolar jets were ejected by the hot component of St 2-22 with an average velocity of about 1700 km/s., Comment: 18 pages, 7 figures, 4 tables, accepted for publication in Acta Astronomica

EE Cep is a unique system in which a Be star is eclipsed by a dark dusty disk, making this star similar to the famous epsilon Aur in many respects. The depth and the duration of the EE Cep eclipses change to a large extent. The last two eclipses were observed in the framework of extensive international campaigns. The joint analysis of these campaigns data and historical photometry, enabled us to propose a model of this system, which implies a disk precession with a period approximately 11-12 times larger than the orbital period. This model predicts that the forthcoming eclipse should be among the deepest observed, reaching about 2 mag. The next eclipse approaches - the photometric minimum should occur around August 23, 2014. Here we would like to announce a new, third international campaign with purpose to verify the disk precession model and to put more constraints on the physical parameters of this system., Comment: 6 pages, 4 figures, accepted in IBVS

We present the results of 45 transit observations obtained for the transiting exoplanet HAT-P-32b. The transits have been observed using several telescopes mainly throughout the YETI network. In 25 cases, complete transit light curves with a timing precision better than $1.4\:$min have been obtained. These light curves have been used to refine the system properties, namely inclination $i$, planet-to-star radius ratio $R_\textrm{p}/R_\textrm{s}$, and the ratio between the semimajor axis and the stellar radius $a/R_\textrm{s}$. First analyses by Hartman et al. (2011) suggest the existence of a second planet in the system, thus we tried to find an additional body using the transit timing variation (TTV) technique. Taking also literature data points into account, we can explain all mid-transit times by refining the linear ephemeris by 21ms. Thus we can exclude TTV amplitudes of more than $\sim1.5$min., Comment: MNRAS accepted; 13 pages, 10 figures

The study of symbiotic systems is of considerable importance in our understanding of binary system stellar evolution in systems where mass loss or transfer takes place. Elemental abundances are of special significance since they can be used to track mass exchange. However, there are few symbiotic giants for which the abundances are fairly well determined. Here we present for the first time a detailed analysis of the chemical composition for the giants in the RW Hya and SY Mus systems. The analysis is based on high resolution (R 50000), high S/N, near-IR spectra. Spectrum synthesis employing standard LTE analysis and atmosphere models was used to obtain photospheric abundances of CNO and elements around the iron peak (Sc, Ti, Fe, and Ni). Our analysis reveals a significantly sub-solar metallicity, [Fe/H]-0.75}, for the RW Hya giant confirming its membership in the Galactic halo population and a near-solar metallicity for the SY Mus giant. The very low 12C/13C isotopic ratios, 6-10, derived for both objects indicate that the giants have experienced the first dredge-up, Comment: MNRAS in press

We present optical photometric monitoring of KT Eri (Nova Eridani 2009), a He/N very fast nova which oubursted in November 2009. Our observations include BVRcIc brightness estimations as well as monitoring of the rapid brightness variations in V band. The characteristic times of these rapid changes are studied and compared with the observed in other novae., Comment: 4 pages, 1 figure, to appear in Stella Novae: Past and Future Decades conference proceedings, P. A. Woudt & V. A. R. M. Ribeiro (eds), ASPCS

Two observational campaigns were carried out during the eclipses of EE Cep in 2003 and 2008/9 to verify whether the eclipsing body in the system is indeed a dark disk and to understand the observed changes in the depth and durations of the eclipses. Multicolour photometric data and spectroscopic observations at both low and high resolution were collected. We numerically modelled the variations in brightness and colour during the eclipses. We tested models with different disk structure. We considered the possibility of disk precession. The complete set of observational data collected during the last three eclipses are made available to the astronomical community. Two blue maxima in the colour indices were detected during these two eclipses, one before and one after the photometric minimum. The first (stronger) blue maximum is simultaneous with a "bump" that is very clear in all the UBVRI light curves. Variations in the spectral line profiles seem to be recurrent during each cycle. NaI lines always show at least three absorption components during the eclipse minimum and strong absorption is superimposed on the H_alpha emission. These observations confirm that the eclipsing object in EE Cep system is indeed a dark, dusty disk around a low luminosity object. The primary appears to be a rapidly rotating Be star that is strongly darkened at the equator and brightened at the poles. Some of the conclusions of this work require verification in future studies: (i) a complex, possibly multi-ring structure of the disk in EE Cep; (ii) our explanation of the "bump" observed during the last two eclipses in terms of the different times of obscuration of the hot polar regions of the Be star by the disk; and (iii) our suggested period of the disk precession (~11-12 P_orb) and predicted depth of about 2 mag the forthcoming eclipse in 2014., Comment: 43 pages, 17 figures, accepted for publication in A&A

The sudden lengthening of orbital period of VV Cep eclipsing binary by about 1% was observed in the last epoch. The mass transfer and/or mass loss are most possible explanations of this event. The photometric behaviour of AZ Cas, the cousin of VV Cep, suggests that the accretion can occur and could be important in this system, too., Comment: 4 pages, 3 figures, contribution to the conference proceedings - "Interacting binaries: Accretion and Synchronization", 2008, June, 20-26, Nauchny, Crimea, Ukraine

The results of time-resolved observations of SU UMa and U Gem obtained over two-years are presented. Both stars are prototypes of different classes of dwarf novae. We studied brightness variations on different time scales: orbital, QPO and flickering. The multicolor BVRI photometry allows to distinguisch the geometrical and physical sources of these variations., Comment: 4 pages, 2 figures, contribution to the conference proceedings: Interacting binaries: Accretion and Synchronization, 2008, June, 20-26, Nauchny, Crimea, Ukraine

Nova V2491 Cyg was discovered on April 10.72 UT 2008 (Nakano, 2008). Here we present spectrophotometric premises that V2491 Cyg can be a good candidate for recurrent nova (RNe). Its properties are compared to five well known RNe with red dwarf secondaries (U Sco, V394 Cra, T Pyx, CI Aql, IM Nor) and recently confirmed as recurrent nova V2487 Oph (Pagnotta et al.,2008). Photometric $U, B, V, R_C, I_C$ and moderate resolution (R$\sim 1500$) spectral observations of V2491 Cyg were carried out in the Torun Observatory (Poland) between April 14 and May 20 2008., Comment: 2 pages, 1 fugure

We present photometric and spectral observation for four novae: V2362 Cyg, V2467 Cyg, V458 Vul, V2491 Cyg. All objects belongs to the "fast novae" class. For these stars we observed different departures from a typical behavior in the light curve and spectrum., Comment: 4 pages, 4 figures

We present UBVRI photometry of three symbiotic stars ZZ CMi, TX CVn and AG Peg carried out from 1997 to 2008 in Piwnice Observatory near Torun. To search orbital periods of these stars Fourier analysis was used. For two of them, TX CVn and AG Peg, we have confirmed the earlier known periods. For ZZ CMi we found a relatively short period 218.59 days. Assuming, that the orbital period is twice longer (P=437.18 days), the double sine wave in the light curve can be interpreted by ellipsoidal effect., Comment: 4 pages, 4 figures, Conference: Interacting Binaries: Accretion and Synchronization

A new estimation of the orbital period of YY Her on the base of our and published observations is presented. Phased light curves in RI bands show evidently ellipsoidal effect connected with the tidal distortion of the giant surface., Comment: 2 pages, 1 figure, Conference Binaries - key to comprehention of the Universe, Brno, Czech Republik

The photometric and spectroscopic observational campaign organized for the 2008/9 eclipse of EE Cep revealed features, which indicate that the eclipsing disk in the EE Cep system has a multi-ring structure. We suggest that the gaps in the disk can be related to the possible planet formation., Comment: 6 pages, 6 figures, contribution to the conference proceedings: Binaries - Key to Comprehension of the Universe, Brno, Czech Republic, 8-12 Jul. 2009

We present the results of unfiltered and UBVR_cI_c band CCD photometry fast nova V2467Cyg. Our analysis of the data gives two distinct frequencies corresponding to periods of ~3.8h and ~35 min. The observed light curve of V2467 Cyg is typical for an intermediate polar., Comment: 4 pages, 5 figures

We present our multicolour photometric data of the primary and secondary eclipses of OW Gem that took place in 1995, 2002, and 2006, as well as the new radial-velocity data collected since 1993 by R. F. Griffin and A. Duquennoy. The Wilson-Devinney code was used for the simultaneous solution of both photometric and spectroscopic data. A complete set of orbital and physical parameters of the components was obtained. The pair of values, eccentricity e=0.5286 and argument of periastron omega=140.73 degree, give better compatibility of the moment of the secondary minimum with the observations compared to previous estimates.

UBVRI photometry observations of 10 symbiotic stars and related objects obtained in the period 2002-2003 are presented. Analysing differential light curves we found rapid light variations with timescales of tens of minutes and significant amplitudes in the well-known flickerers MWC 560, RS Oph, V407 Cyg and T CrB. MWC 560 and V407 Cyg demonstrate quasi periodic oscillations (QPO) with similar amplitudes and timescales. Flickering and unusual flare in V627 Cas as well as some indications of flickering presence in BX Mon are detected. The existence of 29 minutes oscillations in Z And with an amplitude about 0.02 mag in U band is confirmed. Only one symbiotic star, V471 Per, and both non symbiotic, CI Cam and V886 Her seem to be constant on flickering timescales. Nevertheless, small night to night changes in the brightness of V886 Her were observed as well., Comment: 30 pages, 13 figures, accepted for publication in Acta Astronomica

We present photometric and high- and low-resolution spectroscopic observations of the unusual outburst of V838 Mon. The data were collected at the NAO Rozhen, Bulgaria and at the Torun Observatory, Poland. Analysis of the peculiar behavior of the spectrum of the star is given. The star's radial velocity of +60 km/s is derived. A brief discussion concerning the nature of the object is also given., Comment: 10 pages, 4 figures

We present spectroscopic and photometric observations of the recent peculiar outburst of V838 Mon, carried out at Rozhen and Torun observatories. Our data cover a period of three months beginning just before the second eruption. The evolution of the outburst is divided into four phases. The changes of particular spectral features for each of these phases are shortly discussed., Comment: 4 pages, 3 figures, to appear in the proceedings of "Symbiotic stars probing stellar evolution", R.L.M.Corradi, J.Mikolajewska, T.J.Mahoney eds., 2002, ASP Conference Series, in press