Photometric observations of GD 358 - DB white dwarfs do pulsate

GD 358, a helium atmosphere (DB) white dwarf, is found to be a pulsating variable star. The amplitude of its variations can range up to nearly 0.30 mag, and even though the light curve does not repeat itself, there is a typical interval between its pulse maxima of 600-700 s. There are at least 28 pulsation modes simultaneously excited in GD 358 whose periods range from 142.3 s to 952 s, most of which were present on both nights that the star was observed. Several of the pulsations fall into groups of 4 or 5 modes that are equally spaced in frequency, suggesting that the pulsations are l equals 2 g-modes that have been split by rotation. It is noted that all the pulsating white dwarfs found thus far have been hydrogen atmosphere (DA) white dwarfs. Located in a narrow instability strip on the white dwarf cooling sequence at an effective temperature near 11,000 K, they are known as ZZ Ceti stars. On the basis of theoretical studies, it is predicted that whereas the DB white dwarfs should also pulsate, the pulsations should be driven by helium partial ionization rather than by hydrogen partial ionization as they are in the ZZ Ceti stars and that their instability strip should occur at appreciably higher temperatures. The detection of pulsations in GD 358 justifies these predictions.