Well-defined astrophysical constraints of white dwarfs (WDs), such as on the presence or absence of pulsational variability, are very much needed to refine and develop current models. Because these stars are rather faint and variability periods are mostly below one hour, only a very limited amount of space-based data is currently available for these objects. We present about 68 hours of high-quality ground-based photometric time-series data for twenty-one WDs acquired at five different observatories. No new pulsators were detected but the derived upper limits of variability, which are typically on the order of only a few mmags, provide important input for pulsation models.

We report on an analysis of 308.3 hr of high-speed photometry targeting the pulsating DA white dwarf EC14012-1446. The data were acquired with the Whole Earth Telescope during the 2008 international observing run XCOV26. The Fourier transform of the light curve contains 19 independent frequencies and numerous combination frequencies. The dominant peaks are 1633.907, 1887.404, and 2504.897 mu Hz. Our analysis of the combination amplitudes reveals that the parent frequencies are consistent with modes of spherical degree l = 1. The combination amplitudes also provide m identifications for the largest amplitude parent frequencies. Our seismology analysis, which includes 2004-2007 archival data, confirms these identifications, provides constraints on additional frequencies, and finds an average period spacing of 41 s. Building on this foundation, we present nonlinear fits to high signal-to-noise light curves from the SOAR 4.1 m, McDonald 2.1 m, and KPNO 2 m telescopes. The fits indicate a time-averaged convective response timescale of tau(0) = 99.4 +/- 17 s, a temperature exponent N = 85 +/- 6.2, and an inclination angle of theta(i) = 32 degrees.9 +/- 3 degrees.2. We present our current empirical map of the convective response timescale across the DA instability strip.

PG 1351+489 is one of the 20 DBVs – pulsating helium-atmosphere white dwarf stars – known and has the simplest power spectrum for this class of star, making it a good candidate to study cooling rates. We report accurate period determinations for the main peak at 489.334 48 s and two other normal modes using data from the Whole Earth Telescope (WET) observations of 1995 and 2009. In 2009, we detected a new pulsation mode and the main pulsation mode exhibited substantial change in its amplitude compared to all previous observations. We were able to estimate the star’s rotation period, of 8.9 h, and discuss a possible determination of the rate of period change of (2.0 ± 0.9) × 10−13 s s −1, the first such estimate for a DBV.

We report on the analysis of 436.1 hrs of nearly continuous high-speed photometry on the pulsating DB white dwarf GD358 acquired with the Whole Earth Telescope (WET) during the 2006 international observing run, designated XCOV25. The Fourier transform (FT) of the light curve contains power between 1000 to 4000 microHz, with the dominant peak at 1234 microHz. We find 27 independent frequencies distributed in 10 modes, as well as numerous combination frequencies. Our discussion focuses on a new asteroseismological analysis of GD358, incorporating the 2006 data set and drawing on 24 years of archival observations. Our results reveal that, while the general frequency locations of the identified modes are consistent throughout the years, the multiplet structure is complex and cannot be interpreted simply as l=1 modes in the limit of slow rotation. The high k multiplets exhibit significant variability in structure, amplitude and frequency. Any identification of the m components for the high k multiplets is highly suspect. The k=9 and 8 modes typically do show triplet structure more consistent with theoretical expectations. The frequencies and amplitudes exhibit some variability, but much less than the high k modes. Analysis of the k=9 and 8 multiplet splittings from 1990 to 2008 reveal a long-term change in multiplet splittings coinciding with the 1996 "sforzando" event, where GD358 dramatically altered its pulsation characteristics on a timescale of hours. We explore potential implications, including the possible connections between convection and/or magnetic fields and pulsations. We suggest future investigations, including theoretical investigations of the relationship between magnetic fields, pulsation, growth rates, and convection.

EC14012-1446 is a hydrogen atmosphere (DA) white dwarf pulsator. Its rich pulsation spectrum displays a range of excited modes with complex multiplet structure, in addition to numerous combination frequencies. In April 2008, EC14012-1446 was the primary target of XCOV26. We obtained over 300 hrs of nearly continuous high speed photometry with the goal of using the nonlinear pulse shapes to empirically determine the parameters of the convection zone. The Fourier transform (FT) of the light curve contains power between 1000 to 4000 μHz, with the dominant peak at 1234 μHz. We find 13 independent frequencies distributed in 8 modes, as well as a myriad of combination frequencies. In the following, we present preliminary results and lay the groundwork for future investigation leading to light curve fitting of EC14012-1446.

We present simultaneous multicolor infrared and optical photometry of the black hole X-ray transient XTE J1118+480 during its short 2005 January outburst, supported by simultaneous X-ray observations. The variability is dominated by short timescales, ~10 s, although a weak superhump also appears to be present in the optical. The optical rapid variations, at least, are well correlated with those in X-rays. Infrared JHKs photometry, as in the previous outburst, exhibits especially large-amplitude variability. The spectral energy distribution (SED) of the variable infrared component can be fitted with a power law of slope α = -0.78 ± 0.07, where Fν ∝ να. There is no compelling evidence for evolution in the slope over five nights, during which time the source brightness decayed along almost the same track as seen in variations within the nights. We conclude that both short-term variability and longer timescale fading are dominated by a single component of constant spectral shape. We cannot fit the SED of the IR variability with a credible thermal component, either optically thick or thin. This IR SED is, however, approximately consistent with optically thin synchrotron emission from a jet. These observations therefore provide indirect evidence to support jet-dominated models for XTE J1118+480 and also provide a direct measurement of the slope of the optically thin emission, which is impossible, based on the average spectral energy distribution alone.

This paper identifies the cataclysmic variables that appear in spectra obtained in 2004 as part of the Sloan Digital Sky Survey. Spectra of 41 objects, including seven systems that were previously known (CC Cnc, DW Cnc, PQ Gem, AR UMa, AN UMa, RX J1131.3+4322, and UMa 6) and 34 new cataclysmic variables are presented. The positions and ugriz photometry of all 41 systems are given, as well as additional follow-up spectroscopic, photometric, and/or polarimetric observations of eight of the new systems. The new objects include three eclipsing systems, six with prominent He II emission, and six systems that show the underlying white dwarf.

HR1217 is one of the best-studied rapidly oscillating Ap (roAp) stars, with a frequency spectrum of alternating even- and odd-l modes that are distorted by the presence of a strong, global magnetic field. Several recent theoretical studies have found that within the observable atmospheres of roAp stars the pulsation modes are magneto-acoustic with significant frequency perturbations that are cyclic with increasing frequency. To test these theories a Whole Earth Telescope extended coverage campaign obtained 342 h of Johnson B data at 10-s time resolution for the roAp star HR 1217 over 35 d with a 36 per cent duty cycle in 2000 November-December. The precision of the derived amplitudes is 14 mu mag, making this one of the highest precision ground-based photometric studies ever undertaken. Substantial support has been found for the new theories of the interaction of pulsation with the strong magnetic field. In particular, the frequency jump expected as the magnetic and acoustic components cycle through 2 pi rad in phase has been found. Additionally, comparison of the new 2000 data with an earlier 1986 multisite study shows clear amplitude modulation for some modes between 1986 and 2000. The unique geometry of the roAp stars allows their pulsation modes to be viewed from varying aspect with rotation, yielding mode identification information in the rotational sidelobes that is available for no other type of pulsating star. Those rotational sidelobes in HR1217 confirm that two of the modes are dipolar, or close to dipolar; based on the frequency spacings and Hipparcos parallax, three other modes must be either l = 0 or 2modes, either distorted by the magnetic field, or a mix of m-modes of given l where the mixture is the result of magnetic and rotational effects. A study of all high-speed photometric Johnson B data from 1981 to 2000 gives a rotation period P-rot = 12.4572 d, as found in previous pulsation and photometric studies, but inconsistent with a different rotation period found in magnetic studies. We suggest that this rotation period is correct and that zero-point shifts between magnetic data sets determined from different spectral lines are the probable cause of the controversy over the rotation period. This WET data set is likely to stand as the definitive ground-based study of HR1217. It will be the baseline for comparison for future space studies of HR1217, particularly the MOST satellite observations.

BPM 37093 is the only hydrogen-atmosphere white dwarf currently known which has sufficient mass ( 1.1 M_sun) to theoretically crystallize while still inside the ZZ Ceti instability strip (T_eff 12,000 K). As a consequence, this star represents our first opportunity to test crystallization theory directly. If the core is substantially crystallized, then the inner boundary for each pulsation mode will be located at the top of the solid core rather than at the center of the star, affecting mainly the average period spacing. This is distinct from the "mode trapping" caused by the stratified surface layers, which modifies the pulsation periods more selectively. In this paper we report on Whole Earth Telescope observations of BPM 37093 obtained in 1998 and 1999. Based on a simple analysis of the average period spacing we conclude that a large fraction of the total stellar mass is likely to be crystallized.

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_eff 25,000 K) this transformation is still taking place, allowing asteroseismic tests of the theory. Objective global fitting of our updated double-layered envelope models to recent observations of the pulsating DB star CBS 114, and to existing observations of the slightly cooler star GD 358, lead 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.

We observed the pulsating hydrogen atmosphere white dwarf G 185-32 with the Whole Earth Telescope in 1992. We report on a weighted Fourier transform of the data detecting 18 periodicities in its light curve. Using the Hubble Space Telescope Faint Object Spectrograph time resolved spectroscopy, and the wavelength dependence of the relative amplitudes, we identify the spherical harmonic degree (l) for 14 pulsation signals. We also compare the determinations of effective temperature and surface gravity using the excited modes and atmospheric methods, obtaining T-eff = 11 960 +/- 80 K, log g = 8.02 +/- 0.04 and M = 0.617 +/- 0.024 M-circle dot.

Since pulsating subdwarf B (sdBV or EC14026) stars were first discovered, observational efforts have tried to realize their potential for constraining the interior physics of extreme horizontal branch stars. Difficulties encountered along the way include uncertain mode identifications and a lack of stable pulsation mode properties. Here we report on Feige 48, an sdBV star for which follow- up observations have been obtained spanning more than four years. These observations show some stable pulsation modes.We resolve the temporal spectrum into five stable pulsation periods in the range 340- 380 s with amplitudes less than 1 per cent, and two additional periods that appear in one data set each. The three largest amplitude periodicities are nearly equally spaced, and we explore the consequences of identifying them as a rotationally split l = 1 triplet by consulting a representative stellar model.The general stability of the pulsation amplitudes and phases allows us to use the pulsation phases to constrain the time- scale of evolution for this sdBV star. Additionally, we are able to place interesting limits on any stellar or planetary companion to Feige 48.

We report on the analysis of multisite time-series photometry of the pulsating pre-white dwarf (GW Vir star) PG 1707+427, obtained by the Whole Earth Telescope collaboration. This is the last of the known GW Vir stars without surrounding nebulae to be resolved by multisite data. Successful resolution of the pulsation spectrum resulted from the combination of high signal-to-noise observations with a large telescope and wide coverage in longitude with smaller telescopes. We find a series of 8 pulsation frequencies (along with two nonlinear combination frequencies), and identify 7 of them as part of a sequence of l = 1 modes, with a common period spacing of 23.0 s. This spacing implies that the mass of PG 1707+427 is 0.57 M(circle dot). Preliminary model fits suggest that the mass determined via asteroseismology is consistent with the mass determined from spectroscopy combined with evolutionary tracks.

The pulsating DAV white dwarf G 185-32 was observed with the Whole Earth Telescope in 1992, during XCov8. We briefly report on the weighted Fourier transform of the data obtained.

We present results from a multisite (‘Whole Earth Telescope’) photometric campaign on PG 1336−018, the close eclipsing binary system containing a pulsating subdwarf B (sdB) star. The main part of the campaign (1999 April) resulted in ∼172 h of observations, representing

We have acquired new time series photometry of the two pulsating DB white dwarf stars KUV 05134+2605 and PG 1654+160 with the Whole Earth Telescope. Additional single-site photometry is also presented. We use all these data plus all available archival measurements to study the temporal behaviour of the pulsational amplitudes and frequencies of these stars for the first time. We demonstrate that both KUV 05134+2605 and PG 1654+160 pulsate in many modes, the amplitudes of which are variable in time; some frequency variability of PG 1654+160 is also indicated. Beating of multiple pulsation modes cannot explain our observations; the amplitude variability must therefore be intrinsic. We cannot find stable modes to be used for determinations of the evolutionary period changes of the stars. Some of the modes of PG 1654+160 appear at the same periods whenever detected. The mean spacing of these periods (≈40 s) suggests that they are probably caused by non-radial gravity-mode pulsations of spherical degree ℓ = 1. If so, PG 1654+160 has a mass around 0.6 M⊙. The time-scales of the amplitude variability of both stars (down to two weeks) are consistent with theoretical predictions of resonant mode coupling, a conclusion which might however be affected by the temporal distribution of our data.

HR1217 is a prototypical rapidly oscillating Ap star that has presented a test to the theory of nonradial stellar pulsation. Prior observations showed a clear pattern of five modes with alternating frequency spacings of 33.3 μHz and 34.6 μHz, with a sixth mode at a problematic spacing of 50.0 μHz (which equals 1.5 × 33.3 μHz) to the highfrequency side. Asymptotic pulsation theory allowed for a frequency spacing of 34 μHz, but hipparcos observations rule out such a spacing. Theoretical calculations of magnetoacoustic modes in Ap stars by Cunha (2001) predicted that there should be a previously undetected mode 34 μHz higher than the main group, with a smaller spacing between it and the highest one. The 20th extended coverage campaign of the Whole Earth Telescope (XCov20) has discovered this frequency as predicted by Cunha (2001). Amplitude modulation of several of the pulsation modes between the 1986 and 2000 data sets has also been discovered, while important parameters for modelling the geometry of the pulsation modes have been shown to be unchanged. With stringent selection of the best data from the WET network the amplitude spectrum shows highest peaks at only 50 μmag and formal errors on the determined amplitudes are 14 μmag. Some lessons for future use of WET for the highest precision photometry on bright stars are discussed.