A SPECTROSCOPICALLY PREDICTED NEW ZZ CETI VARIABLE - GD-165

We report the discovery that the DA white dwarf GD 165, recently identified by Becklin and Zuckerman as possibly having a brown dwarf companion, is also a ZZ Ceti variable. The variability of GD 165 was previously predicted from the spectroscopic analysis of Bergeron et al.: GD 165 was found to have atmospheric parameters similar to other ZZ Ceti white dwarfs, namely an effective temperature near 13,000 K and a pure hydrogen atmosphere. The analysis of our two independent photometric observations shows brightness variations of up to 0.1 mag on time scales of 200-1800 s. Fourier analysis of the light curves indicates that, as is true in general for ZZ Ceti variables, multiple periodicities are present on both nights and the period structure of the star can change on a time scale of 2 days. In the first run, Fourier analysis shows a multiperiodic low-frequency spectrum, while on the second run, the low-frequency spectrum is dominated by an ∼1800 s period, which is the longest period ever observed in a ZZ Ceti white dwarf. A high-frequency peak of variable amplitude corresponding to a period of 120 s is present in both runs as well. The time variability of the period structure is interpreted as interaction between pulsation modes, as is seen in other ZZ Ceti variables.