Measuring the Evolutionary Rate of Cooling of ZZ Ceti

We have finally measured the evolutionary rate of cooling of the pulsating hydrogen atmosphere white dwarf ZZ Ceti (R548), as reflected by the 213.13260694 s period. Using 41 years of time-series photometry from 1970 to 2011, we determine its rate of change of period with time to be dP/dt = (3.0 +/- 1.4) x 10(-15) s/s with the O - C method and (3.23 +/- 0.87) x 10(-15) s/s using the nonlinear least squares fit, after correcting for proper motion. We augment the uncertainty to a more realistic value and arrive at the evolutionary cooling rate of (3.2 +/- 1.2) x 10(-15) s/s for ZZ Ceti, consistent with the measurement of (4.19 +/- 0.73) x 10(-15) s/s for G117-B15A.