The DAV (ZZ Ceti) stars, with effective surface temperatures of about 12,000 K, are the coolest of the several classes of pulsating white dwarf and pre-white dwarf stars. From the available observational data, it is clear that although these objects typically pulsate simultaneously in a number of nonradial g-modes with periods of about several hundred seconds, these periods are remarkably stable. This is in accord with the theory of these geometrically- constrained almost completely degenerate objects. Nevertheless, it is expected that evolutionary cooling effects will lead to detectable period changes over a sufficiently long time scale. One object, G 117-B15A, has revealed definite signs of a period change in its principal pulsation mode, with the aid of two decades of high-speed photometry. The object L19-2 now has a high-speed photometry data base covering 20 years, including a successful Whole Earth Telescope run in 1995. An analysis of the period stability of this object will be presented.
A Half Century of Stellar Pulsation Interpretation
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