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Astron. Astrophys. 329, L13-L16 (1998)

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1. Introduction

ZZ Ceti or DAV stars are normal DA white dwarfs (pure H atmospheres) which show light variations produced by non-radial g- mode pulsations caused by convective driving (Brickhill 1991) in the hydrogen partial ionization zone. All pulsating DA stars are found between [FORMULA]  11,160 K and 12,460 K (Bergeron et al. 1995) on the white dwarf cooling sequence, and this narrow strip is known as the ZZ Ceti instability strip. The ZZ Ceti stars are mutliperiodic and therefore the astereoseismological study of this class can provide a detalied understanding of white dwarf structure and evolution (Kleinman et al. 1994; Pfeiffer et al. 1996). The ZZ Ceti stars have also been used to estimate the evolutionary time of these stars on the white dwarf cooling sequence (Kepler et al. 1991). By using the theoretical age-luminosity relation to interpret the observed white dwarf luminosity function it is possible to obtain a minimum age for the local Galactic disk (Winget et al. 1987; Wood 1995; Oswalt et al. 1996). The Whole Earth Telescope (WET; Nather et al. 1990) has proved to be a powerful tool for probing the structure of variable stars using temporal spectroscopy of complex light curves - see e.g. Kepler & Bradley (1995) and references therein for a review on white dwarf seismology - and the discovery of new targets for future seismological studies is important to increase our knowledge of stellar structure and evolution.

Bradley (1995) lists 24 known ZZ Ceti stars and recently Vauclair et al. (1997) and Jordan et al. (1997) discovered 3 new DAV stars increasing the number of known ZZ Ceti stars to 27. Of all known ZZ Ceti stars only seven are in the Southern hemisphere, indicating the especially incomplete surveying for Southern ZZ Ceti stars. Therefore, we have selected all DA white dwarf stars from the McCook & Sion (1987) catalogue with colors near or within the ranges [FORMULA] (Fontaine et al. 1982), [FORMULA] (Fontaine et al. 1985), and [FORMULA] (Greenstein 1982), to monitor for variability.

For the past 10 years there has been a serious search to find which stellar parameters are sufficient to place a variable star in the ZZ Ceti instability strip. Originally, it was believed that [FORMULA] was the unique parameter since all then known DA stars inside the instability strip were variable (Fontaine et al. 1985). However, Dolez et al. (1991), Kepler and Nelan (1993), Kepler et al. (1995), and recently Giovannini (1996) found several non-variable DA stars inside the strip; these results indicate another parameter should distinguish between variables and non-variables, or that the instability strip is not "pure". Using the theoretical results by Bradley & Winget (1994) that the blue edge temperature of the ZZ Ceti instability strip depends on the stellar mass, Kanaan (1996) showed that the observations agree.

Using the colors as indicators, we have already discovered one DAV, BPM 37093 (Kanaan et al. 1992), the most massive known DAV at [FORMULA] (Bergeron et al. 1995), and now this paper reports the second DAV star discovered, BPM 24754. It was originally selected by its color and later we determined its [FORMULA] and [FORMULA] spectroscopically. In the meantime, extensive time series photometry was obtained and during two observing runs we found it to vary.

In this paper, we show the light curves, their Fourier spectra, and we also determine the fundamental parameters [ [FORMULA], [FORMULA], mass] of BPM 24754. Finally, we discuss its location in relation to the observed ZZ Ceti instability strip.

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© European Southern Observatory (ESO) 1998

Online publication: November 24, 1997