High-Frequency Stellar Oscillations.VI. R548, a Periodically Variable White Dwarf

The white dwarf R548 is a periodic variable with a dominant period of 212.864 + 0.031 sec and a secondary period of 273.0 + 0.6 sec. The amplitude of the former is 0.01 mag, while that of the latter fluctuates between 0.001 and 0.01 mag on a time scale of <24 hours. Light curves are given for both variations. This star and the other confirmed periodically variable white dwarfs, HL Tau 76 and G44-32, lie near the lower junction of the DA boundary and the blackbody line in the [(U - B), (B - V) i-diagram, but other stars in this region of the two-color plane ap ear quiescent. The variations of these white dwarfs cannot be readily interpreted as pulsations, and e need for other physical models is discussed.