Abstract
G117-B15A is a pulsating DA white dwarf, or ZZ Ceti-type star. Using high-speed photometry accumulated over the 5 year interval from 1975 to 1980, the unusually complex variations of its light curve have been disentangled. G117-B15A has six pulsation modes simultaneously excited with periods of 107.6 s, 119.8 s, 126.2 s, 215.2 s, 271.0 s, and 304.4 s. The 215.2 s pulsation has the largest semiamplitude, 0.022 mag, of the six pulsations and dominates the light curve. The upper limit to the rate of change of the period of this large amplitude pulsation is less than 7.8 x 10 to the -14th s/s at the 68% confidence level. The pulsations of the ZZ Ceti stars are usually considered to be the nonradial g-mode pulsations. The above upper limit to the rate of change of the period is consistent with this idea, but the ratios of the periods can be understood more easily if the pulsations are r-modes than if they are g-modes.
Published Version
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