We have been studying the pulsation periods for the DAV variable white dwarf star G117-B15A for the last 27 years, showing it is an extremely stable pulsator and measuring the rate of pulsation period, directly related to the cooling of the white dwarf. On the other hand, the pulsating DBV GD358 shows changes in amplitudes and frequencies on every timescale we studied it since 1990. The phases also change at least as fast as a monthly timescale.
