Watching ZZ Ceti evolve

We report preliminary results from our analysis of the stability of periods observed in the pulsating hydrogen atmosphere white dwarf ZZ Ceti (R548) based on observations that span 37 years from 1970 to 2007. We determine the rate of change of period with time to be dP/dt (0.8±1.9)x10-15 s/s using the O-C method and dP/dt (4.3±1.2)x10-15 s/s using the direct non-linear least squares fit NLSPDOT for the dominant period 213.13260643 s after correcting for proper motion. We do not claim either of these values as a measurement at this time, but hope to arrive at a conclusive result in the near future with more observations. The reduced uncertainty for both methods shows the improvement we obtained over the previous evolutionary constraint on ZZ Ceti (Mukadam et al. 2003). These dP/dt values are consistent within uncertainties with the measurement of dP/dt = (3.57 ±0.82)x10-15 s/s for the period 215.2 s observed in another pulsating white dwarf G117-B15A (Kepler et al. 2005). Using the 213 s triplet spacing of 4μHz, we compute the rotation period of ZZ Ceti to be 1.5 days.