Whole earth telescope observations of the white dwarf G29-38: Phase variations of the 615 second period

Using an extensive set of high-speed photometric observations obtained with the Whole Earth Telescope network, we show that the complex light curve of the ZZ Ceti (DAV) star G29-38 is dominated by a single, constant amplitude period of 615 s during the time span of our observations. The pulse arrival times for this period exhibit a systematic variation in phase readily explained by light-travel time effects produced by reflex orbital motion about an unseen companion. Our best-fit model to the observations indicates a highly eccen­ tric orbit, a period of 109 ± 13 days and a minimum mass of 0.5 Mʘ for the companion. Radial velocity variations predicted by this model are not observed, however, nor are these phase variations seen in another independent pulsation, so the origin of the phase variation remains a mystery. Any model involving intrinsic pulsation mechanisms must explain the large (~200 s) phase change with no corresponding change in pulsation amplitude, and it shape, which mimics quite exactly the effects of binary orbital motion.