Complications to the Planetary Hypothesis for GD 66

Pulsating hydrogen atmosphere white dwarf stars (DAVs, also known as ZZ Ceti stars) are some of the most stable optical clocks in the universe, some exhibiting rates of period change on the order of a few microseconds per year (10^-15 s/s). Periodic modulation in times of arrival of these stable non-radial pulsations could indicate the presence of an unseen companion. Using this method, the white dwarf GD 66 exhibited evidence of a 4.5-year modulation in its 302.8 s mode commensurate with a roughly 2 Jupiter-mass planet, making the star a planet-host candidate (Mullally et al. 2008). Additional observations have followed the times of arrival of this 302.8 s mode, which continue to show a roughly 4 year modulation. Additionally, we see modulation on a similar timescale in the times of arrival of the 271.1 s mode in the star. However, the trend for each mode is nearly exactly out of phase with the other. This strongly complicates the planetary hypothesis for GD 66, since an external companion would perturb all modes in the star identically in phase. We also discuss additional pulsating white dwarf stars that similarly show this peculiar behavior.