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Published online by Cambridge University Press: 12 April 2016
There are now twelve ZZ Ceti stars known and their properties are summarized by Nather (1978) and McGraw (1977, 1979). They have normal DA spectra and their colors range from a B-V of 0.18 mag to 0.22 mag. This places them on the HR diagram in a region which is an extension to the white dwarfs of the Cepheid instability strip. This would suggest that the cause of their pulsation could be analogous to the κ and γ mechanisms of the Cepheids except for the fact that the periods of the ZZ Ceti stars range from 200 to 1200 seconds, three orders of magnitude larger than the radial pulsation periods for white dwarfs.
also at Department of Physics, Arizona State University, Tempe, AZ.
Supported in part by NSF Grant AST77-23190 to Arizona State University.