The final stages of stellar evolution leading to formation and cooling of white dwarf stars, include six different classes of pulsating stars. Along the line of evolution from intermediate-mass progenitors, there are first the Planetary Nebula Nucleus Variables (PNNVs) and GW Virginis pulsators. These two classes of pre-white dwarf pulsators inhabit the high-temperature entry-point into the cooling track for C/O-core white dwarfs, and some members are among the hottest stars known. They also include the second-richest pulsator after the Sun (GW Vir itself, the first great triumph of asteroseismology), and another fascinating star that emits three times more energy in neutrinos than in light.
Next in line are the variable helium-atmosphere white dwarfs (DBVs), just below the DB gap between 29,000 K and 21,000 K. This was the first class of variable stars predicted to exist prior to their discovery. The prototype, GD 358, was after GW Vir the second significant success story of Asteroseismology; it provided the first asteroseismological measurements of distance and magnetic field strength, and it now serves as a significant test-bed for new mode-identification techniques using high-order combination frequencies and time-series spectroscopy.
At the cool end of the white-dwarf cooling track are the hydrogen-atmosphere pulsators, the DAV or ZZ Ceti stars. These are so far the most numerous among the classes I will discuss, but also in some ways the most frustrating because individual stars generally show few periods-making mode-identification very difficult. Attempts have been made in recent years to explain their group properties by considering the periods of many stars at once, but more data are needed to prove the worth of this interesting idea. The DAVs have other uses, however: at least one is so massive that theory predicts it should have a crystallized core. Asteroseismology can test and calibrate this forty-year old prediction, but so far results are inconclusive.
Finally, two new classes of pulsating "pre-white dwarf" were discovered recently: the short-period (p-mode) sdB stars, and just this year the long-period (g-mode) sdBVs. These was the second case variable stars whose existence was predicted prior to discovery, and they occupy the extreme end of horizontal branch, just above the "helium ZAMS." These stars are not massive enough to make the trip up the AGB, and will apparently fall directly onto the cooling track for helium-core white dwarfs. I will discuss the first tantalizing results obtained in studies of the short-period sdBVs. The discovery, properties, and promise of the long-period sdBVs form the subject of a separate review talk at this conference.

 
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