On the statistics of ZZ Ceti stars

Results are summarized of a continuing survey for additional pulsating white dwarfs. Particular attention is given to the color distributions of stars investigated for variability. It is noted that (in terms of the Greenstein G-R color index) among the hydrogen-rich white dwarfs, only variable stars have thus far been found in the G-R range from -0.45 to -0.38. This color range corresponds to an interval of effective temperatures from 13,000 K to 11,000 K. This observational result is seen as implying that the vast majority, and probably all, of the hydrogen-rich white dwarfs evolve to become ZZ Ceti variables during an evolutionary phase of their lifetimes. Completeness arguments and a discussion of possible selection effects corroborate this conclusion. The consequences of this result for theoretical studies of ZZ Ceti stars are briefly discussed.