Progress and Problems in the Study of the Pulsating White Dwarf Stars

We currently know of at least three distinct classes of degenerate pulsating variable stars; they occur with a practically uniform spacing in the log of the effective temperature and span nearly the full sweep of the white dwarf cooling sequence in the H-R diagram. The hottest of these variable stars are the pulsating PG 1159-035 stars. Extremely hot, compact, stars, they appear to be contracting and cooling on their way to becoming white dwarf stars. These proto-white dwarfs have photometric properties similar to the pulsating white dwarf stars, and are reviewed separately in these proceedings by A. N. Cox. The two remaining classes of compact pulsating variables are found in relatively narrow instability strips occupying distinct portions of the white dwarf cooling sequence after the constant radius, purely cooling, phase has been reached. These two classes of variable stars are the topic of this review. In this work I will use the concise notation introduced by Sion et al. (1983) to indicate the two classes of variables: the ZZ Ceti’s, and the pulsating DB white dwarfs, become simply the DAV, and the DBV stars, respectively.