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Photometric studies of the type SU UMa dwarf nova NY Her were made during 2017 and 2020 at the Crimean Astrophysical Observatory and at the Sanglokh Observatory over a total of 77 nights. These observations covered two superoutbursts, four normal outbursts, and the quiescent state between them. In 2017 the supercycle was 62 days, and the cycle, 13-14 days. Positive superhumps with an average period of 0.07594 days and a variable amplitude not exceeding 0m.15 were observed during the superoutbursts. In the quiescent state between outbursts, as well as in the ascending and descending branches of the normal outbursts, negative superhumps with an average period of 0.071269 days were recorded. Their amplitude ranged over 0m.3-1m.5 in the quiescent state. The period of the negative superhumps manifested cyclical increases between normal outbursts, which is consistent with the prediction of a cyclical change in the radius of the accretion disk in terms of the thermal-tidal theory of the instability.

IW And stars are a recently recognized subgroup of dwarf novae which are characterized by (often repetitive) slowly rising standstills terminated by brightening, but the exact mechanism for this variation is not yet identified. We have identified BO Cet, which had been considered as a novalike cataclysmic variable, as a new member of IW And stars based on the behavior in 2019-2020. In addition to this, the object showed dwarf nova-type outbursts in 2020-2021, and superhumps having a period 7.8% longer than the orbital one developed at least during one long outburst. This object has been confirmed as an SU UMa-type dwarf nova with an exceptionally long orbital period (0.1398 d). BO Cet is thus the first cataclysmic variable showing both SU UMa-type and IW And-type features. We obtained a mass ratio (q) of 0.31-0.34 from the superhumps in the growing phase (stage A superhumps). At this q, the radius of the 3:1 resonance, responsible for tidal instability and superhumps, and the tidal truncation radius are very similar. We interpret that in some occasions this object showed IW And-type variation when the disk size was not large enough, but that the radius of the 3:1 resonance could be reached as the result of thermal instability. We also discuss that there are SU UMa-type dwarf novae above q=0.30, which is above the previously considered limit (q~0.25) derived from numerical simulations and that this is possible since the radius of the 3:1 resonance is inside the tidal truncation radius. We constrained the mass of the white dwarf larger than 1.0Msol, which may be responsible for the IW And-type behavior and the observed strength of the He II emission. The exact reason, however, why this object is unique in that it shows both SU UMa-type and IW And-type features is still unsolved., 10 pages, 7 figures, supporting information (sibo.pdf), accepted for publication in PASJ

EM Cyg is an eclipsing dwarf nova with an orbital period above the period gap of cataclysmic variables. Based on the whole outburst of EM Cyg observed by the Transiting Exoplanet Survey Satellite (TESS) from 2019 July 18–August 14, the evolution of quasi-periodic oscillations (QPOs) in EM Cyg was investigated. A QPO with a period of about 1000 s was found during both outburst and quiescence. In addition, we found another QPO with a period of about 500 s, which was present during the outburst but disappeared as the outburst ended. QPOs should be caused by the activity of the accretion disc. Using the data obtained during the outburst, we analysed the characteristics of the light curves and found that the eclipsing depth increased and the light-minimum moment earlier during the outburst. In addition, with the new eclipse times obtained by TESS and from our observations, an O − C diagram for this system was produced. According to the O − C analysis, the orbital period is increasing at a rate of $\dot{P} = 1.66(\pm 0.36) \times 10^{-8}\, \mathrm{d\,yr^{-1}}$. There is a cyclic variation in the O − C curve that might be caused by the light-traveltime effect. A third body may exist in the EM Cyg system with the mass $m_3\sin {i{^{\prime }}} = 0.097(\pm 0.000\,35) \, \mathrm{M}_{\odot }$.

We present a complete dynamical study of the intermediate polar and dwarf nova cataclysmic variable GK Per (Nova Persei 1901) based on a multi-site optical spectroscopy and $R$-band photometry campaign. The radial velocity curve of the evolved donor star has a semi-amplitude $K_2=126.4 \pm 0.9 \, \mathrm{km}\,\mathrm{s}^{-1}$ and an orbital period $P=1.996872 \pm 0.000009 \, \mathrm{d}$. We refine the projected rotational velocity of the donor star to $v_\mathrm{rot} \sin i = 52 \pm 2 \, \mathrm{km}\,\mathrm{s}^{-1}$ which, together with $K_2$, provides a donor star to white dwarf mass ratio $q=M_2/M_1=0.38 \pm 0.03$. We also determine the orbital inclination of the system by modelling the phase-folded ellipsoidal light curve and obtain $i=67^{\circ} \pm 5^{\circ}$. The resulting dynamical masses are $M_{1}=1.03^{+0.16}_{-0.11} \, \mathrm{M}_{\odot}$ and $M_2 = 0.39^{+0.07}_{-0.06} \, \mathrm{M}_{\odot}$ at $68$ per cent confidence level. The white dwarf dynamical mass is compared with estimates obtained by modelling the decline light curve of the $1901$ nova event and X-ray spectroscopy. The best matching mass estimates come from the nova light curve models and an X-ray data analysis that uses the ratio between the Alfv\'en radius in quiescence and during dwarf nova outburst., Comment: 15 pages, 6 figures, 8 tables, accepted for publication in MNRAS

Photometric studies of the SU UMa-type dwarf nova IRXS J161659.5+620014 have been made on the 2.6-m and 1.25-m telescopes at the Crimean Astrophysical Observatory during 2017 and 2018 in the quiescent state of the system, during normal outbursts, and during superoutbursts. It is found that the interval between adjacent normal outbursts was about 11 days during 2017. Observations in the quiescent state during 2017 yielded an orbital period of 0.06888(2) days. Observations during 2018 manifested good reproducibility of the characteristics of the superoutbursts compared to those of previous years. A refined period of 0.071044(15) days was obtained for the positive superhumps in stage “C” and their color indices were determined.

Author(s): Ofek, Eran O; Soumagnac, Maayane; Nir, Guy; Gal-Yam, Avishay; Nugent, Peter; Masci, Frank; Kulkarni, Shri R | Abstract: ABSTRACT Variable sources probe a wide range of astrophysical phenomena. We present a catalogue of over 10 million variable source candidates found in Data Release 1 (DR1) of the Zwicky Transient Facility (ZTF). We perform a periodicity search up to a frequency of 160 d−1, and we classify the light curves into erratic and smooth variables. We also present variability indicators and the results of a periodicity search, up to a frequency of 5 d−1, for about 1 billion sources in the ZTF-DR1 light curve data base. We present several new short-period (l90 min) candidates, and about 60 new dwarf nova candidates, including two candidate eclipsing systems. Both the 10 million variables catalogue and ∼1 billion source catalogue are available online in catsHTM format.

We present X-ray observations of novae V2491 Cyg and KT Eri about 9 years post-outburst, of the dwarf nova and post-nova candidate EY Cyg, and of a VY Scl variable. The first three objects were observed with XMM-Newton, KT Eri also with the Chandra ACIS-S camera, V794 Aql with the Chandra ACIS-S camera and High Energy Transmission Gratings. The two recent novae, similar in outburst amplitude and light curve, appear very different at quiescence. Assuming half of the gravitational energy is irradiated in X-rays, V2491 Cyg is accreting at $\dot{m}=1.4\times10^{-9}-10^{-8}M_\odot/yr$, while for KT Eri, $\dot{m}, 14 pages, 12 figures, 6 tables

Results of the first multicolor photometric observations of the unusual dwarf nova V2466 Cygni during its 2003 and 2019 superoutbursts are presented. The total amplitude of the 2019 superoutburst in the $$R_{c}$$ band is 7.4 magnitudes. A single rebrightening was observed at the end of the 2019 superoutburst. While analyzing the $$O-C$$ diagram for the superhumps, we identified pronounced stages A, B, and C corresponding to the evolution of their period. The mean superhump period is $$P=0\overset{\textrm{d}}{.}08143(3)$$ for both superoutbursts; the superhump period derivative is $$P_{\textrm{dot}}=10.4(1.4)\times 10^{-5}$$ in stage B. We argue for the classification of V2466 Cyg as a member of a small group of objects combining the characteristics of SU UMa and WZ Sge dwarf novae and propose an explanation for the evolutionary status of these objects. As an additional result, we have discovered and studied 20 new variable stars in the neighborhoods of V2466 Cyg.

We present the X-ray analysis of dwarf nova EY Cyg using the 45 ks \textit{XMM-Newton} observatory archival data obtained in quiescence. We find orbital modulations in X-rays. We simultaneously fitted EPIC pn, MOS1 and MOS2 data using a model for interstellar medium absorption (\textit{tbabs}) and a multi-temperature plasma emission model with a power-law distribution of temperatures (CEVMKL) as expected from low accretion rate quiescent dwarf novae. The \textit{XMM-Newton} EPIC spectra of the source yields a maximum temperature $kT_{\rm max}$ $\sim$ 14.9$^{+3.3}_{-2.2}$ keV with an unabsorbed X-ray flux and luminosity of (1.8--2.0) $\times$ 10$^{-12}$ ergs$^{-1}$ cm$^{-1}$ and (8.7--9.7) $\times$ 10$^{31}$ ergs$^{-1}$, respectively, in the energy range 0.1 to 50 keV. There is 3--4 sigma excess at energies below 0.5 keV, we model the excess using \textit{MEKAL}, POWERLAW and BBODY models and favor the model \textit{MEKAL} which is physical. According to previous studies, the secondary in this system is thought to be a K-type star which may radiate in the soft X-ray region. The fit with an additive \textit{MEKAL} model gives a temperature of $kT$ $\sim$ 0.1 keV with an unabsorbed X-ray flux and luminosity of (2.7--8.8) $\times$ 10$^{-14}$ ergs$^{-1}$ cm$^{-1}$ and (1.3--4.2) $\times$ 10$^{30}$ ergs$^{-1}$, respectively, for the companion star. Based on the results from the timing and spectral analysis, we highly suggest that the secondary of EY Cyg is a K-type star., 7 pages, 3 figures, 3 tables. Accepted for publication in a Special Issue of Advances in Space Research, entitled "Nova Eruptions, Cataclysmic Variables and Related Systems: observational vs theoretical challenges in the 2020 era", edited BY S. Balman and P. A. Shea

We present the results of the first year of searching for polar candidates within the 3BS (3-Band Survey) program using medium-band filters. Observational data for 84 selected objects from the catalog of cataclysmic variables of the CRTS DR1 sky survey were obtained while implementing the program. A dwarf nova during its outburst and a polar were detected. The available archival data are analyzed. The dwarf nova is of the U Gem or Z Cam type and contains a massive secondary component with $$M_{2}=0.94\pm 0.04M_{\odot}$$ . For the polar found we have estimated the orbital period $$P_{O}=0\overset{d}{.}054437$$ and calculated the magnetic field strength of the white dwarf $$B\approx 32$$ MG.

The spectra of disc-dominated cataclysmic variables (CVs) often deviate from the spectra of accretion disc models; in particular, the Balmer jump and absorption lines are found to be shallower in the observations than in the models. We carried out a combined ultraviolet-optical spectral analysis of two dwarf novae: UZ Ser in outburst, decline, and quiescence, and CY Lyr on the rise to outburst and in outburst. We fit the Balmer jump and absorption lines, the continuum flux level and slope by adjusting the accretion rate, inclination, and disc outer radius. For both systems we find an accretion rate $\dot{M} \approx 8 \times 10^{-9}M_\odot$/yr in outburst, and $\dot{M} \approx 2-3 \times 10^{-9}M_\odot$/yr for the rise and decline phases. The outer disc radius we derive is smaller than expected ($R_{\rm disc} \approx 0.2a$, where $a$ is the binary separation), except during late rise (for CY Lyr) where $R_{\rm disc}=0.3a$. UZ Ser also reveals a 60,000~K white dwarf. These results show that during a dwarf nova cycle the radius of the disc is the largest just before the peak of the outburst, in qualitative agreement with the disc instability model for dwarf nova outbursts. We suspect that an additional emitting component (e.g. disc wind) is also at work to reduce the slope of the continuum and size of the Balmer jump and absorption lines. We stress that both the outer disc radius and disc wind need to be taken into account for more realistic disc modeling of CVs., Comment: 19 pages, 13 figures, submitted to MNRAS

The envelopes accreted by white dwarf stars from their hydrogen-rich companions1 experience thermonuclear-powered runaways2,3 observed as classical nova eruptions4,5 peaking at 105–106 solar luminosities6–9. Virtually all nova progenitors—‘nova-like variables’—exhibit high mass transfer rates to their white dwarfs before and after an eruption10. Surprisingly, 10–1,000 times lower mass transfer rate11 binaries, exhibiting accretion-powered ‘dwarf nova’ outbursts12, exist at identical orbital periods. Nova shells surrounding dwarf novae13–16 demonstrate that at least some novae metamorphize into dwarf novae17,18, though the mechanisms and timescales governing mass transfer rate variations are poorly understood. Here, we report simulations of the multi-Gyr evolution of novae modelling every eruption’s thermonuclear runaway, mass and angular momentum losses, feedback due to irradiation and variable mass transfer rate, and orbital size and period changes. These feedback-dominated simulations reproduce the observed range of mass transfer rates at a given orbital period, with large and cyclic kyr–Myr timescale changes. They also demonstrate Myr-long deep hibernation (complete stoppage of mass transfer), but only in short-period binaries; that initially different binaries converge to become nearly identical systems; low-mass-transfer-rate dwarf novae occasionally generate novae; and that the masses of white dwarfs decrease monotonically, but only slightly while their red dwarf companions are consumed. Cataclysmic variables—a binary pairing of a white dwarf and a hydrogen-rich donor star—experience mass transfer and other complex interactions. This numerical simulation by Hillman et al. models in particular the feedback between the stellar pair, and succeeds in reproducing many of the observed characteristics of cataclysmic variables.

We report a long-term (1961-2017) study of the eclipse times in the dwarf nova WZ Sagittae, in an effort to learn its rate of orbital-period change. Some wiggles with a time scale of 20-50 years are apparent, and a connection with the 23-year interval between dwarf-nova eruptions is possible. These back-and-forth wiggles dominate the O-C diagram, and prevent a secure measurement of the steady rate of orbital-period change. The line, it is drawn, the curse, it is cast. The slow one now will later be fast... For the times, they are a-changin'. - Dylan (1963), PDF, 17 pages, 1 table, 5 figures; submitted, PASP; more info at http://cbastro.org/

I performed photometric observations of the cataclysmic variable candidate ZTF17aaaeefu and discovered very deep eclipses. The observations were obtained over 8 nights covering 7 months. The eclipse profile is similar to the eclipse profiles observed in other cataclysmic variables. During the observations, ZTF17aaaeefu showed brightness changes resembling dwarf nova outbursts and quiescent states. When ZTF17aaaeefu was bright (16.0-16.2 mag), the average eclipse depth was 2.50+/-0.18 mag. When ZTF17aaaeefu was faint (17.2-17.4 mag), the average eclipse depth was 1.45+/-0.06 mag. However, these differences in the eclipse depth may be mainly caused by the contaminating effect of three faint stars around ZTF17aaaeefu. In both cases, the average width of the prominent parts of the eclipses was the same and was about 30 min. Due to the large coverage of observations, I measured the orbital period with high precision, Porb=0.18821155+/-0.00000014 d. I derived the eclipse ephemeris, the validity time of which is 700 years in accordance with the precision of the orbital period. This ephemeris can be used for future studies of the orbital period changes. Because ZTF17aaaeefu has a long orbital period, this cataclysmic variable is of interest for determining the masses of its stellar components. In future radial velocity measurements, my precise eclipse ephemeris may be useful for determining the orbital phases., 14 pages, 6 figures, accepted for publication in Astrophysics and Space Science