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The motivation, techniques and performance of the ground-based photometric follow-up of transit detections by the CoRoT space mission are presented. Its principal raison d'êtrearises from the much higher spatial resolution of common ground-based telescopes in comparison to CoRoT's cameras. This allows the identification of many transit candidates as arising from eclipsing binaries that are contaminating CoRoT's lightcurves, even in low-amplitude transit events that cannot be detected with ground-based obervations. For the ground observations, “on” – “off” photometry is now largely employed, in which only a short timeseries during a transit and a section outside a transit is observed and compared photometrically. CoRoTplanet candidates' transits are being observed by a dedicated team with access to telescopes with sizes ranging from 0.2 to 2 m. As an example, the process that led to the rejection of contaminating eclipsing binaries near the host star of the Super-Earth planet CoRoT-7b is shown. Experiences and techniques from this work may also be useful for other transit-detection experiments, when the discovery instrument obtains data with a relatively low angular resolution.

Context.PG 1159-035, a pre-white dwarf with $T_{{\rm eff}}$$\simeq$140 000 K, is the prototype of both two classes: the PG 1159 spectroscopic class and the DOV pulsating class. Previous studies of PG 1159-035photometric data obtained with the Whole Earth Telescope (WET) showed a rich frequency spectrum allowing the identification of 122 pulsation modes. Analyzing the periods of pulsation, it is possible to measure the stellar mass, the rotational period and the inclination of the rotation axis, to estimate an upper limit for the magnetic field, and even to obtain information about the inner stratification of the star.

Context.The variable pre-white dwarf PG 1159 stars (GW Vir) are g-mode non-radial pulsators. Asteroseismology puts strong constraints on their global parameters and internal structure. PG 0122+200 defines the red edge of the instability strip and its evolutionary timescale is predicted to be dominated by neutrino emission. Its study offers the opportunity to better understand the instability mechanism and to validate the physics of the neutrino production in dense plasma.

We studied the class of DB white dwarf stars, using re-calibrated UV spectra for thirty four DBs obtained with the IUE satellite. By comparing the observed energy distributions with model atmospheres, we simultaneously determine spectroscopic distances (d), effective temperature ($T_{{\rm eff}}$), and surface gravities ($\log g$). Using parallax measurements and previous determinations of $T_{{\rm eff}}$and $\log g$from optical spectra, we can study whether the atmospheres of eleven DB stars are consistent with pure He or have a small amount of H contamination. We also report on our observations of seventeen stars with $T_{{\rm eff}}$close to the DB instability strip through time series photometry and found them to be non variable within our detection limits.

This paper analyses the Whole Earth Telescope observations of HL Tau 76, the first discovered pulsating DA white dwarf. The star was observed during two Whole Earth Telescope campaigns. It was a second priority target during the XCOV13 campaign in 1996 and the first priority one during the XCOV18 campaign in 1999. The 1999 campaign reached 66.5% duty cycle. With a total duration of 18 days, the frequency resolution achieved is 0.68 μHz. With such a frequency resolution, we were able to find as many as 78 significant frequencies in the power spectrum, of which 34 are independent frequencies after removal of all linear combinations. In taking into account other frequencies present during the 1996 WET campaign and those present in earlier data, which do not show up in the 1999 data set, we find a total of 43 independent frequencies. This makes HL Tau 76 the richest ZZ Ceti star in terms of number of observed pulsation modes. We use those pulsation frequencies to determine as much as possible of the internal structure of HL Tau 76. The pulsations in HL Tau 76 cover a wide range of periods between 380 s and 1390 s. We propose an identification for 39 of those 43 frequencies in terms of $\ell=1$and $\ell=2$non-radial g-modes split by rotation. We derive an average rotation period of 2.2 days. The period distribution of HL Tau 76 is best reproduced if the star has a moderately “thick” hydrogen mass fraction log $q_{\rm H} \geq -7.0$. The results presented in this paper constitute a starting point for a detailed comparison of the observed periods with the periods calculated for models as representative as possible of HL Tau 76.

Extensive photometric multisite campaigns of the δScuti variable FG Vir are presented. For the years 2003 and 2004, 926 h of photometry at the millimag precision level were obtained. The combinations with earlier campaigns lead to excellent frequency resolution and high signal/noise. A multifrequency analysis yields 79 frequencies. This represents a new record for this type of star. The modes discovered earlier were confirmed.
Pulsation occurs over a wide frequency band from 5.7 to 44.3 c/d with amplitudes of 0.2 mmag or larger. Within this wide band the frequencies are not distributed at random, but tend to cluster in groups. A similar feature is seen in the power spectrum of the residuals after 79 frequencies are prewhitened. This indicates that many additional modes are excited. The interpretation is supported by a histogram of the photometric amplitudes, which shows an increase of modes with small amplitudes. The old question of the “missing modes” may be answered now: the large number of detected frequencies as well as the large number of additional frequencies suggested by the power spectrum of the residuals confirms the theoretical prediction of a large number of excited modes.
FG Vir shows a number of frequency combinations of the dominant mode at 12.7162 c/d ($m = 0$) with other modes of relatively high photometric amplitudes. The amplitudes of the frequency sums are higher than those of the differences. A second mode (20.2878 c/d) also shows combinations. This mode of azimuthal order $m = -1$is coupled with two other modes of $m = +1$.

The helium-atmosphere (DB) white dwarfs are commonly thought to be the descendants of the hotter PG 1159 stars, which initially have uniform He/C/O atmospheres. In this evolutionary scenario, diffusion builds a pure He surface layer which gradually thickens as the star cools. In the temperature range of the pulsating DB white dwarfs ($T_{\rm eff} \sim 25\,000$K) this transformation is still taking place, allowing asteroseismic tests of the theory. We have obtained dual-site observations of the pulsating DB star CBS 114, to complement existing observations of the slightly cooler star GD 358. We recover the 7 independent pulsation modes that were previously known, and we discover 4 new ones to provide additional constraints on the models. We perform objective global fitting of our updated double-layered envelope models to both sets of observations, leading to determinations of the envelope masses and pure He surface layers that qualitatively agree with the expectations of diffusion theory. These results provide new asteroseismic evidence supporting one of the central assumptions of spectral evolution theory, linking the DB white dwarfs to PG 1159 stars.

A high-accuracy multisite campaign was carried out from 2002 January to May with a photometric coverage of 398 h at five observatories. The concentration on a few selected sites gives better consistency and accuracy than collecting smaller amounts from a larger number of sites. 23 frequencies were detected with a high statistical significance. 6 of these are new. The 17 frequencies found in common with the 1992–1995 data are the modes with highest amplitudes. This indicates that the pulsation spectrum of FG Vir is relatively stable over the ten-year period. Two frequencies have variable amplitudes and phases from year to year as well as during 2002. These were both found to be double modes with close frequencies. For the mode at 12.15 c/d this leads to an apparent modulation with a time scale of ~129 d. The close frequencies at 12.15 c/d are composed of a radial and a nonradial mode, suggesting a similarity with the Blazhko Effect seen in RR Lyrae stars.

We report the discovery of a new eclipsing system less than one arcminute south of the pulsating DB white dwarf KUV 05134+2605. The object could be identified with the point source 2MASS J0516288+260738published by the Two Micron All Sky Survey. We present and discuss the first light curves as well as some additional colour and spectral information. The eclipse period of the system is 1.29 d, and, assuming this to be identical to the orbital period, the best light curve solution yields a mass ratio of $m_2/m_1=0.11$, a radius ratio of $r_2/r_1\approx 1$and an inclination of 74°. The spectral anaylsis results in a $T_{\rm eff}=4200$K for the primary. On this basis, we suggest that the new system probably consists of a late K + Brown dwarf (which would imply a system considerably younger than ≈0.01 Gyr to have $r_2/r_1\approx 1$), and outline possible future observations.

We present results from an international spectroscopic and photometric campaign on the δScuti star AV Cet. The star has a rich and complex pulsation spectrum, and we find 7 individual frequencies, with evidence for many more present below our detection limit. We investigate the prospects for mode identification in fast rotators, using several different techniques. We compare the methods and conclude that although no single technique can give unambiguous mode identification, the collective evidence does allow some conclusions to be drawn, suggesting the presence of one radial mode at 14.598 d-1. During the campaign we found the star HD 9139to be a variable. From our photometry we find evidence for a variability time scale around 1 day, but we cannot find an unambiguous interpretation for its light variations.

By pursuing the goal to find new variables in the COROT field–of–view we characterised a sample of stars located in the lower part of the instability strip. Our sample is composed of stars belonging to the disk population in the solar neighbourhood. We found that 23% of the stars display multiperiodic light variability up to a few mmag in amplitude, i.e., easily detectable on a single night of photometry. $uvby\beta$photometry fixed most of the variables in the middle of the instability strip and high–resolution spectroscopy established that they have $v\sin i >100$km s-1. An analysis of the Rodriguez & Breger ([CITE]) sample (δSct stars in the whole Galaxy) shows slightly different features, i.e., most δSct stars have a 0.05–mag redder $(b-y)_0$index and lower $v\sin i$values. Additional investigation in the open cluster NGC 6633 confirms the same incidence of variability, i.e., around 20%. The wide variety of pulsational behaviours of δSct stars (including unusual objects such as a variable beyond the blue edge or a rapidly rotating high–amplitude pulsator) makes them very powerful asteroseismic tools to be used by COROT. Being quite common among bright stars, δSct stars are suitable targets for optical observations from space.

Detailed elemental abundances were derived for twenty bona fide λBootis as well as two MK standard stars. Other than LTE abundances for ten elements (including C and O), NLTE values for Na were determined. The group of λBootis stars consists of non-magnetic, Population I, late B to early F-type dwarfs with a typical abundance pattern (Fe-peak elements being underabundant whereas C, N, O and S being almost solar abundant). Since classification resolution spectroscopy in the optical domain is not capable of determining the abundance of the light elements, a detailed abundance analysis is the ultimate test for the membership of an object to this group. Another important point is the detection of apparent spectroscopic binary systems in which two solar abundance objects mimic one metal-weak star, as proposed as a working hypothesis by Faraggiana & Bonifacio (1999). From twenty program stars we are able to confirm or establish the membership for nine objects (HD 23258, HD 36726, HD 40588, HD 74911, HD 84123, HD 91130, HD 106223, HD 111604 and HD 290799). Five stars (HD 90821, HD 98772, HD 103483, HD 108765 and HD 261904) can be definitely ruled out as being members of the λBootis group whereas no unambiguous decision can be drawn for another six stars (HD 66684, HD 105058, HD 120500, HD 141851, HD 201184 and HD 294253). One very important result is the apparent overabundances found for Na which cannot be explained by accretion or mass-loss alone.

Generally, chemical peculiarity found for stars on the upper main sequence excludes δScuti type pulsation (e.g. Ap and Am stars), but for the group of λBootis stars it is just the opposite. This makes them very interesting for asteroseismological investigations. The group of λBootis type stars comprises late B- to early F-type, Population I objects which are basically metal weak, in particular the Fe group elements, but with the clear exception of C, N, O and S. The present work is a continuation of the studies by Paunzen et al. ([CITE], [CITE]), who presented first results on the pulsational characteristics of the λBootis stars. Since then, we have observed 22 additional objects; we found eight new pulsators and confirmed another one. Furthermore, new spectroscopic data (Paunzen [CITE]) allowed us to sort out misidentified candidates and to add true members to the group. From 67 members of this group, only two are not photometrically investigated yet which makes our analysis highly representative. We have compared our results on the pulsational behaviour of the λBootis stars with those of a sample of δScuti type objects. We find that at least 70% of all λBootis type stars inside the classical instability strip pulsate, and they do so with high overtone modes (Q< 0.020 d). Only a few stars, if any, pulsate in the fundamental mode. Our photometric results are in excellent agreement with the spectroscopic work on high-degree nonradial pulsations by Bohlender et al. ([CITE]). Compared to the δScuti stars, the cool and hot borders of the instability strip of the λBootis stars are shifted by about 25 mmag, towards smaller $(b-y)_0$. Using published abundances and the metallicity sensitive indices of the Geneva 7-colour and Strömgren $uvby\beta$systems, we have derived [ Z] values which describe the surface abundance of the heavier elements for the group members. We find that the Period-Luminosity-Colour relation for the group of λBootis stars is within the errors identical with that of the normal δScuti stars. No clear evidence for a statistically significant metallicity term was detected.

We present the results of a large international spectroscopic campaign on the δScuti star BN Cnc. Combining observations from five observatories taken over more than two weeks, we calculate line indices of the Hαline. A line index is the integrated line flux in a software filter divided by the continuum flux. We demonstrate that this can be used in combination with simultaneous photometry to classify the oscillation modes. We recover all the frequencies also found from photometry and assign likely mode identifications, which differ slightly from previously published values, but are found to be consistent with simple models. The difference in identification is found to have very little effect on the derived luminosity and temperature.

We report that the intermediate amplitude δScuti star V 1162 Ori has changed its main pulsational period in the course of the year 2000. This new period change falls in a sequence of period changes observed during the last 5 years. While the average amplitude value of all our new data, 63 mmag, fits a cyclic amplitude variation suggested by Arentoft et al. (2001), splitting the data up in smaller subsets discloses significant deviations from regularity, with stretches of constant amplitude during short intervals of time. The new data show that the amplitude of one of the secondary frequencies, f2, has in 3 years dropped from more than 3 mmag to now about 1 mmag, and that the previously obtained f5probably is a 1 d-1alias of the real frequency. We present the newly acquired times of minimum and maximum light as support for subsequent observing campaigns.

We present the results of multisite observations of the δScuti star V 1162 Ori. The observations were done in the period October 1999 -May 2000, when 18 telescopes at 15 observatories were used to collect 253 light extrema during a total of 290 hours of time-series observations. The purpose of the observations was to investigate amplitude and period variability previously observed in this star, and to search for low-amplitude frequencies. We detect, apart from the main frequency and its two first harmonics, four additional frequencies in the light curves, all with low amplitudes (1-3 mmag). Combining the present data set with data obtained in 1998-99 at ESO confirms the new frequencies and reveals the probable presence of yet another pulsational frequency. All five low-amplitude frequencies are statistically significant in the data, but at least one of them (f5) suffers from uncertainty due to aliasing. Using colour photometry we find evidence for a radial main frequency (f1), while most or all low-amplitude frequencies are likely non-radial. We show that the main frequency of V 1162 Ori has variable amplitude and period/phase, the latter is also displayed in the O-C diagram from light extrema. The amplitude variability in our data is cyclic with a period of 282 d and a range of nearly 20 mmag, but earlier amplitude values quoted in the literature cannot be explained by this cyclic variation. O-C analysis including data from the literature show that the period of V 1162 Ori displays a linear period change as well as sudden or cyclic variations on a time scale similar to that of the amplitude variations. ]

Context.The ??Bootis stars are a chemically peculiar subgroup of main sequence A/F stars. Several of these fall into the instability strip of the ??Scuti stars and the whole spectroscopic group shows a high incidence of pulsational variability.Aims.We investigated the pulsational behaviour of the ??Bootis star 15?And (HD?221756) to determine as many intrinsic mode frequencies as possible for asteroseismic applications.Methods.The star was included as a secondary target for three telescopes during a campaign devoted to the ??Cep stars 12 and 16?Lac, due to its proximity to these targets. 302.8?h of time-resolved photometry spanning 161.2?d were acquired and analyzed.Results.Our frequency analysis revealed f1= 24.7887(2) c/d as the main pulsation frequency, with an amplitude of only 1.9(1)?mmag. The residual periodogram suggests the presence of additional frequencies in an interval from?20 to 25?c/d, with very low amplitudes. We succeeded in detecting a second frequency f2= 22.2580(9) with an amplitude of?0.6(1) mmag. Low-frequency light variations of the comparison star HD?220575 were revealed as well.Conclusions.The pulsational behaviour of 15 And as a typical ? Boo-type star is compared with that of???Scuti stars with normal abundances and with metallic-line ??Scuti stars. We discuss the value of?? Boo stars in understanding accretion and mixing in pulsating stars. The difficulties in the selection of reliable comparison stars for such high-precision research are pointed out.

The star HD 205 has been discovered to be a new $\delta$ Sct-type pulsator through simultaneous $uvby$ photometric observations carried out during the autumn of 2001. A few H$_{\beta}$-Crawford measurements were also collected. The calibration of the Strömgren photometric indices places this star in the middle part of the $\delta$ Sct instability strip. The behaviour of the light curves is shown to be very complex with a lot of frequencies necessary to fit the data. The data in the bands $v$, b and y were analyzed separately to confirm detected pulsations and then combined using a weighting scheme to maximize the signal-to-noise ratio. Nine frequencies were detected with the present data, but several more are probably present. The majority of the detected frequencies are due to nonradial low order p-modes as is commonl in $\delta$ Sct variables.

Context.The Kepler spacecraft is providing time series of photometric data with micromagnitude precision for hundreds of A-F type stars.

Context.Studying stellar pulsations in open clusters offers the possibility of performing ensemble asteroseismology. The reasonable assumption that the cluster members have the same age, distance, and overall metallicity aids in the seismic modelling process and tightly constrains it. Therefore it is important to identify open clusters with many pulsators.

Context.The βCephei stars and slowly pulsating B (SPB) stars are massive main sequence variables. The strength of their pulsational driving strongly depends on the opacity of iron-group elements. As many of those stars naturally occur in young open clusters, whose metallicities can be determined in several fundamental ways, it is logical to study the incidence of pulsation in several young open clusters.