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Asteroseismology offers the possibility of probing stellar interiors and testing evolutionary and seismic models. Precise photometry and spectroscopy obtained during multi-site campaigns on young open clusters allows discovering rich samples of pulsating stars and using them in a simultaneous seismic modelling called ensemble asteroseismology. The aim of this study is to obtain the age of the open cluster NGC 6910 by means of ensemble asteroseismology of the early-type pulsating members, to derive their stellar parameters, and to classify the excited modes. We used time-series analysis, performed photometric and spectroscopic mode identification, and calculated grids of evolutionary and seismic models to apply the procedure of ensemble asteroseismology for nine pulsating members of NGC 6910. With two iterations of the procedure of ensemble asteroseismology applied to nine pulsating stars we derived an age of 10.6$^{+0.9}_{-0.8}$ Myr for NGC 6910. Of the nine pulsating stars examined in the paper, eight are $\beta$ Cep stars, including three that are hybrid $\beta$ Cep and slowly pulsating B-type (SPB) pulsators, and one is an SPB star. Interestingly, the least massive $\beta$ Cep star, NGC 6910-38, has a mass of about 5.6 M$_\odot$. The present theory does not predict unstable $p$ modes in B-type stars with such a low mass. The $g$ modes with relatively high frequencies ($>3.5$ d$^{-1}$), observed in three members of the cluster, are also stable according to seismic modelling. Both findings pose a challenge for theoretical calculations and prompt a revision of the opacities. The procedure of ensemble asteroseismology was found to be successful for NGC 6910 and $\chi$ Per on the basis of pulsating B-type stars and can therefore be applied to other young open clusters that are rich in such stars., Comment: 22 pages, 24 figures. Caution: arXiv abstract is shorter than in the article

NGC 6910 is the northern hemisphere open cluster known to be rich in $\beta$ Cephei-type stars. Using four-season photometry obtained in Bia\l k\'ow (Poland) and Xinglong (China) observatories, we performed variability survey of NGC 6910. As the result, we found over 100 variable stars in the field of the cluster, including many stars showing variability due to pulsations and binarity. Thanks to the spectroscopic observations, we also detected changes in the profiles of spectral lines of $\beta$ Cep stars, caused by pulsations., Comment: 3 pages, 1 figure, XXXVIII Polish Astronomical Society Meeting

We present the results of a photometric variability survey in the young open cluster Stock 14 and the surrounding fields. In total, we detected 103 variable stars of which 88 are new discoveries. We confirm short-period, low-amplitude light variations in two eclipsing members of the cluster, HD 101838 and HD 101794. In addition, we find two new beta Cephei stars of which one, HD 101993, is also a member. The sample of pulsating cluster members is supplemented by one multimode slowly pulsating B-type star and several single-mode candidates of this type. The other pulsating stars in our sample are mostly field stars. In particular, we found fourteen delta Scuti stars including one gamma Dor/delta Sct hybrid pulsator. From our UBV photometry we derived new parameters of Stock 14: the mean reddening E(B-V) = 0.21 +/- 0.02 mag, the true distance modulus, 11.90 +/- 0.05 mag, and the age, 20 +/- 10 Myr. Finally, we use the new photometry to analyze changes of the 6.322-d orbital period of the bright eclipsing binary and the member of the cluster, V346 Cen. In addition to the known apsidal motion, we find that another effect, possibly light-time effect in a hierarchical system of a very long orbital period, affects these changes. The updated value of the period of apsidal motion for this system amounts to 306 +/- 4 yr. The open cluster Stock 14 was found to be a fairly good candidate for successful ensemble asteroseismology., Comment: 30 pages, 14 figures. Accepted for publication in Acta Astronomica

We summarize our ground-based program of spectroscopic and photometric observations of the asteroseismic targets of the Kepler space telescope. We have already determined atmospheric parameters, projected velocity of rotation, and radial velocity of 62 Kepler asteroseismic targets and 33 other stars in the Kepler field of view. We discovered six single-lined and two double-lined spectroscopic binaries, we determined the interstellar reddening for 29 stars in the Kepler field of view, and discovered three delta Sct, two gamma Dor and 14 other variable stars in the field of NGC 6866., Comment: 4 pages, 4 figures. Stellar Pulsation: Challenges for Theory and Observation, Santa Fe, NM (USA), May 31 - June 5, 2009

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

We present results of a photometric study of the young southern open cluster Stock 14. This cluster is known to contain two eclipsing systems with presumed beta Cephei components, HD 101794 and HD 101838. We confirm variability due to pulsations and eclipses in both targets and announce the discovery of other variable stars in the observed field., Comment: 3 pages, 4 figures, Astronomische Nachrichten, HELAS IV Conference, Arecife, Lanzarote, Feb 2010, submitted

As a result of the variability survey in Chi Persei and NGC6910, the number of Beta Cep stars that are members of these two open clusters is increased to twenty stars, nine in NGC6910 and eleven in Chi Persei. We compare pulsational properties, in particular the frequency spectra, of Beta Cep stars in both clusters and explain the differences in terms of the global parameters of the clusters. We also indicate that the more complicated pattern of the variability among B type stars in Chi Persei is very likely caused by higher rotational velocities of stars in this cluster. We conclude that the sample of pulsating stars in the two open clusters constitutes a very good starting point for the ensemble asteroseismology of Beta Cep-type stars and maybe also for other B-type pulsators., Comment: 4 pages, Astronomische Nachrichten, HELAS IV Conference, Arecife, Lanzarote, Feb 2010, submitted

We present the ground-based activities within the different working groups of the Kepler Asteroseismic Science Consortium (KASC). The activities aim at the systematic characterization of the 5000+ KASC targets, and at the collection of ground-based follow-up time-series data of selected promising Kepler pulsators. So far, 36 different instruments at 31 telescopes on 23 different observatories in 12 countries are in use, and a total of more than 530 observing nights has been awarded. (Based on observations made with the Isaac Newton Telescope, William Herschel Telescope, Nordic Optical Telescope, Telescopio Nazionale Galileo, Mercator Telescope (La Palma, Spain), and IAC-80 (Tenerife, Spain). Also based on observations taken at the observatories of Sierra Nevada, San Pedro Martir, Vienna, Xinglong, Apache Point, Lulin, Tautenburg, Loiano, Serra la Nave, Asiago, McDonald, Skinakas, Pic du Midi, Mauna Kea, Steward Observatory, Mt Wilson, Bialkow Observatory of the Wroclaw University, Piszkesteto Mountain Station, Observatoire de Haute Provence, and Centro Astronomico Hispano Aleman at Calar Alto. Based on data from the AAVSO International Database.), Comment: 4 pages, Astronomische Nachrichten, HELASIV Conference, Lanzarote, February 2010, submitted

CONTEXT: Recent progress in the seismic interpretation of field beta Cep stars has resulted in improvements of the physics in the stellar structure and evolution models of massive stars. Further asteroseismic constraints can be obtained from studying ensembles of stars in a young open cluster, which all have similar age, distance and chemical composition. AIMS: To improve our comprehension of the beta Cep stars, we studied the young open cluster NGC 884 to discover new B-type pulsators, besides the two known beta Cep stars, and other variable stars. METHODS: An extensive multi-site campaign was set up to gather accurate CCD photometry time series in four filters (U, B, V, I) of a field of NGC884. Fifteen different instruments collected almost 77500 CCD images in 1286 hours. The images were calibrated and reduced to transform the CCD frames into interpretable differential light curves. Various variability indicators and frequency analyses were applied to detect variable stars in the field. Absolute photometry was taken to deduce some general cluster and stellar properties. RESULTS: We achieved an accuracy for the brightest stars of 5.7 mmag in V, 6.9 mmag in B, 5.0 mmag in I and 5.3 mmag in U. The noise level in the amplitude spectra is 50 micromag in the V band. Our campaign confirms the previously known pulsators, and we report more than one hundred new multi- and mono-periodic B-, A- and F-type stars. Their interpretation in terms of classical instability domains is not straightforward, pointing to imperfections in theoretical instability computations. In addition, we have discovered six new eclipsing binaries and four candidates as well as other irregular variable stars in the observed field., Comment: Accepted for publication in Astronomy and Astrophysics, 21 pages, 14 figures, 4 tables. The full appendix is available at http://www.ster.kuleuven.be/~sophies/Appendix.pdf (74 MB, 169 pages, 343 figures, 1 table)

We present results of variability search in the field of the young open cluster NGC 1502. Eight variable stars were discovered. Of six other stars in the observed field that were suspected for variability, we confirm variability of two, including one beta Cep star, NGC 1502-26. The remaining four suspects were found to be constant in our photometry. In addition, UBVI photometry of the well-known massive eclipsing binary SZ Cam was obtained. The new variable stars include: two eclipsing binaries of which one is a relatively bright detached system with an EA-type light curve, an alpha2 CVn-type variable, an SPB candidate, a field RR Lyrae star and three other variables showing variability of unknown origin. The variability of two of them is probably related to their emission in Halpha, which has been measured by means of the alpha index obtained for 57 stars brighter than V=16 mag in the central part of the observed field. Four other non-variable stars with emission in Halpha were also found. Additionally, we provide VI photometry for stars down to V=17 mag and UB photometry for about 50 brightest stars in the observed field. We also show that the 10-Myr isochrone fits very well the observed color-magnitude diagram if a distance of 1 kpc and mean reddening, E(V-I)=0.9 mag, are adopted.

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 report the discovery of 19 variable stars and two blue-stragglers in the field of the open cluster NGC6866. Three of the variable stars we classify as delta Sct, two, as gamma Dor, four, as WUMa, two, as ellipsoidal variables, and one, as an eclipsing binary. Seven stars show irregular variability. Two of the pulsators, a delta Sct star NGC 6866-29 and a gamma Dor star NGC6866-21, are multiperiodic. From an analysis of proper motions, we conclude that the delta Sct stars, one of the gamma Dor stars and both blue-stragglers are very probable members of the cluster. The position on the color-magnitude diagram of seven other variables suggests that they also belong to the cluster. The eclipsing binary, which we discover to be a new high-velocity star, and the seven irregular variables are non-members. Then, we discuss in detail the age and metallicity of open clusters that host gamma Dor stars and we show that none of these parameters is correlated with the number of gamma Dor stars in cluster., Comment: 20 pages, 13 figures, 1 on-line table

We present time-series radial-velocity and photometric observations of a solar-type double-lined eclipsing binary star (V12) near the turnoff in the old open cluster NGC188. We use these data to determine the spectroscopic orbit and the photometric elements for V12. From our analysis we determine accurate masses (Mp=1.103+-0.007Msun, Ms=1.081+-0.007Msun) and radii (R =1.424+-0.019Rsun, Rs=1.373+-0.019Rsun) for the primary (p) and secondary (s) binary components. We adopt a reddening of E_(B-V)=0.087 for NGC188, and derive component effective temperatures of 5900+-100K and 5875+-100K, respectively, for the primary and secondary stars. From their absolute dimensions, the two components of V12 yield identical distance moduli of Vo-MV=11.24+-0.09, corresponding to 1770+-75pc. We determine an age of 6.2+-0.2Gyr for V12 and NGC188, from a comparison with theoretical isochrones in the mass-radius diagram. This age is independent of distance, reddening, and color-temperature transformations. We use isochrones from Victoria-Regina (VRSS) and Yonsei-Yale (Y2) with [Fe/H]=-0.1 and [Fe/H]=0.0. From the solar metallicity isochrones, an age of 6.4Gyr provides the best fit to the binary components for both sets of models. For the isochrones with [Fe/H]=-0.1, ages of 6.0Gyr and 5.9Gyr provide the best fits for the VRSS and Y2 models, respectively. We use the distance and age estimates for V12, together with best estimates for the metallicity and reddening of NGC188, to investigate the locations of the corresponding VRSS and Y2 isochrones relative to cluster members in the color-magnitude diagram. Plausible changes in model metallicity and distance to better match the isochrones to the cluster sequences, result in a range of ages for NGC188 that is more than 3 times that resulting from our analysis of V12., Comment: 36 pages, 10 figures (1 color), 9 tables

From the Fourier analysis of the catalogue of ~200,000 variable candidates in the OGLE-II Galactic fields, we found a sample of about 230 short-period low-amplitude variable stars. From their position in the colour-magnitude diagram and the observed periods (multiple in most cases), we identify the stars as a mixture of Beta Cephei and Delta Scuti stars. Beta Cephei stars from this sample are located in the Galactic disk at distances from 3 to 6 kpc. Many of them shows large range of the excited periods, an indication of high metallicity. We estimate that even a half of the sample of 230 short-period variables we found, can be Beta Cephei stars. The periods alone, however, are rarely sufficient to distinguish between both types of pulsators. We point out, how this can be done observationally., Comment: 2 pages, 2 figures, to appear in the proceedings of the "Stellar Pulsation and Evolution" conference, Monte Porzio Catone, June 2005

Searching for main-sequence pulsators, we analyzed photometry of ~200,000 variable star candidates from the OGLE-II Galactic fields, finding 193 high-amplitude Delta Scuti stars. This doubles the number of known stars of this type. The MACHO data, available for half of stars, were also analyzed. In our sample of the HADS stars, we found 50 multiperiodic objects, including 39 that have period ratios in the range of 0.76-0.80, an indication of the radial fundamental and first-overtone pulsation. We discuss the resulting Petersen diagram for these stars in view of the period ratios predicted by models. Except for stars showing pulsations in the radial fundamental mode and first overtone, we find the evidence for higher radial overtones and non-radial modes in the analyzed sample of multiperiodic HADS stars., Comment: 4 pages, 3 figues, to appear in the proceedings of the "Stellar Pulsation and Evolution" conference, Monte Porzio Catone, June 2005