STEPHI is a photometry network that aims to obtain data to reach a better understanding of the physics of stellar interiors by seismology of δ Scuti stars. For ten years now STEPHI has been harvesting multisite rapid photometry data of δ Scuti stars on the main sequence. With 12 δ Scuti stars observed during three-week, three-site campaigns, STEPHI has produced a significant amount of the data available on δ Scuti stars for seismological study of this stage of evolution. The STEPHI data have already been basis of several theoretical interpretations (see http://dasgal.obspm.fr/˜stephi). Here we focus on some aspects of our observational strategy and results.

Pulsation is ubiquitous among chemically normal A-type stars, but comparatively rare among chemically peculiar Am and Ap stars of the same temperature range. The conventional explanation for this is that diffusion produces the surface abundance anomalies in the Am and Ap stars, and also drains He from the He-II ionisation zone, thus quenching the κ-mechanism that drives δ Scuti pulsation. The pulsating Am and Ap stars exhibit dichotomous pulsation characteristics. The Am stars (and related stars) exhibit low-overtone δ Scuti pulsation, with amplitudes ranging from a few mmag to 0.1 mag. The pulsating Ap stars exhibit high-overtone pulsation with periods in the range 6-16 min and Johnson B semi-amplitudes typically ≤ 5 mmag. These stars are referred to as rapidly oscillating Ap stars, or ‘roAp’ stars (the see review by Martinez & Kurtz 1995).

A spectroscopic survey of roAp stars has been initiated in Vienna in order to determine their fundamental astrophysical parameters and abundances. We report here on our attempt to confirm and elaborate an atmospheric peculiarity recently discovered (Ryabchikova et al. 1999) which should allow to identify roAp stars with a single spectrum.

We have used the 2D-Coudé spectrograph of the 2.7-m telescope at McDonald Observatory at a resolution of 60,000. We use an iodine cell which provides us with permanent wavelength reference.

γ Equ was observed for a period of approximately 6 hours. The signal to noise ratio was variable due to the presence of clouds. A typical value for the “good” spectra is 80.

The use of a large detector provides us with complete spectral coverage from 5,000 to 6,000 Å (the region where I2 lines are most visible and useful as a wavelength reference). This allowed us to expand over our previous work analyzing the pulsations of γ Equulei through the use of spectroscopy.

We acquired multisite observations of the newly discovered rapidly oscillating Ap star HD 122970. A frequency analysis shows the presence of two independent modes, one singlet and one equally spaced triplet. By applying the oblique pulsator model and asymptotic theory we infer that the triplet originates from a distorted dipole mode and that the singlet most probably corresponds to a radial mode.

We report on the pulsation characteristics of λ Bootis stars as a result of a survey initiated to increase the number of known λ Bootis stars and to improve the definition of this group of peculiar stars.

We present the main results of a photometric and spectroscopic study of the Λ Boo star 29 Cyg carried out from 1995 – 1998.

We present the current status of the ongoing search for B-type pulsators in the double cluster h and χ Persei.

Based on new analyses of light curves of SX Phœnicis variables in globular clusters and on results taken from the literature, an overview of oscillation patterns in this subgroup of δ Scuti variables is given. Belonging to the Blue Straggler stars, they are expected to have a more complicated life history than “normal” δ Scuti stars in the field or in open clusters. The overall picture is that the patterns found in this SX Phe group seem to be similar to the patterns that are well established for standard field δ Set stars.

The preliminary results of the CCD search for variable stars in the very young open cluster NGC 6823 are presented. About ten new variable stars were discovered. The most interesting finding is that of the two pre-main sequence (PMS) δ Scuti-type variables. Because the evolutionary rate of period change is large at the PMS stage, these stars could be used in the future for testing it observationally.

We examine the pulsation properties for 13 members of the pulsating subdwarf B (sdBV, or EC 14026) class of stars. By looking at the pulsation structure of an entire class of stars, it may be possible to determine the various modes of pulsations (O’Brien 1998, Kleinman 1995). Unfortunately, when we examine the ensemble of pulsation periods in EC 14026 stars, we are only able to discern a simple correlation between pulsation period and gravity, and not any structure that might help with mode identification. So we can only report on the lack of structure in the pulsation periods and present some of the work underway, which we hope will identify modes in the future.

Oscillations in white dwarfs of hydrogen or helium envelopes are believed to be excited close to the surface, where convective energy transport dominates the stellar luminosity. The convective motion in these stars is fast and can respond instantaneously to the pulsation state. In this limit, we find the convective envelope to be the seat of mode excitation because it acts as an insulating blanket with respect to the perturbed flux that enters it from below. This retaining of the flux leads to driving. Driving exceeds radiative damping provided ωτc ≥ 1, where ω is the radian frequency of the mode and τc ≈ 4τth with τth being the thermal time constant evaluated at the base of the convective envelope. We follow Brickhill (1991) in naming this mechanism as ‘convective driving’. We also studied the dynamical interaction between turbulent convection and pulsation. In the limit of fast convection, turbulent damping inside the convective region is negligible, while that coming from the overshoot region is more significant. I discuss the application of ‘convective driving’ in other types of pulsating stars.

We are searching for the coolest white dwarf stars in the galactic disk and halo. The Sloan survey, in due course, will identify an enormous number of new white dwarf stars which will better define the white dwarf luminosity function—an important tool for understanding the age and history of the stellar population of the galaxy. The broadband filter data obtained in the digital photometry phase of the survey will not permit identification of the most interesting of these, the coolest white dwarf stars. This is because the cool main sequence and subdwarf stars become indistinguishable from the white dwarfs in the various colorcolor diagrams. We have interference filters designed to separate out these classes of objects. We have obtained photometry of test fields to complement the Sloan data and identify the population of cool white dwarf stars. These data will ultimately resolve the controversies, based for the most part on small-number statistics, of the location of the turndown in the white dwarf luminosity function for the disk. If the halo is significantly older than the disk, we will find a second peak in the white dwarf luminosity function, at lower luminosities than the disk turndown. Our data will provide the first meaningful constraints on the location of the turndown in the halo white dwarf luminosity function.

We have created a grid of DAV (ZZ Ceti) pulsation models to test whether ensemble observations of DAV pulsation periods can be used to determine the nature of the DAV interiors.

We report the discovery of a new variable DA white dwarf (ZZ Ceti): PG1541+650, identified as a potential ZZ Ceti candidate in the Hamburg Quasar Survey.

With a time-series CCD photometry survey, we have demonstrated clearly that the observed red edge for the ZZ Ceti stars instability strip at 11,000 K is not an observational selection effect. Previous surveys for variability among hydrogen atmosphere white dwarfs at around 11,000 K have been carried out using high speed photometry which suffers from variable extinction effects that start becoming important at periods of 15 minutes. In our survey we constantly monitor the sky brightness as well as one or more comparison stars. This is done through the same color filter, therefore minimizing adverse effects of differential extinction. The fact that the theoretical red edge should be around 8,000 K remains, but effects not included in the theory, especially convection-aulsation interaction, could explain it.

We have obtained time-series spectra for the pulsating hot subdwarf (sdB) PG1605+072. Previous time-series photometry of this star has shown maximum amplitude variations of ∼0.1 mag and at least 50 periods. The pulsator has the largest amplitude and longest periods of all the pulsating sdBs (or EC 14026 stars) discovered so far, and appears to be unusual in its class. Preliminary results of a search for velocity variations are presented here. With these variations, along with equivalent width measurements, oscillation mode identification should be possible.

We have used the rate of change of pulsation period for the hot (DOV) pre-white dwarf PG1159–035 and the cool (DAV) white dwarf G117–B15A to measure their evolutionary time scales. We show that, for any multiperiodic star, we must take into account the effect of all pulsations simultaneously on the times of maximum of the pulsations to get reliable measurements of periods and phases.

Although the frequencies of the pulsating white dwarfs are extremely stable, the evolutionary cooling of these geometrically constrained objects should lead to an observable period change over a sufficiently long time base. Direct measurement of a period change rate for the cooler white dwarfs should provide fundamental data on the core composition of these stellar remnants, and hence their cooling time scales. Progress on attempts to measure the period change rates of two white dwarfs is reported.

We are searching for pulsations in cool (< 6000 K) white dwarfs (WDs), hoping to apply asteroseismological techniques to improve our understanding of their structure and the physical processes inside them. This information is important as we use cool WDs to estimate the lower limit of the age of the Galactic disk. Within a spectroscopic and photometric survey of 110 cool WDs by Bergeron, Ruiz, & Legget, we find 28 candidates with appropriate effective temperatures, masses, and chemical compositions for possible pulsations in nonradial g modes with periods similar to those we observe in DAVs. So far, we have observed 4 candidates, but have found no evidence of large variation.