The Peculiar Pulsations of PY Vulpeculae

The pulsating white dwarf star PY Vul (G185-32) exhibits pulsation modes with peculiar properties that set it apart from other variable stars in the ZZ Ceti (variable DA white dwarf [DAV]) class. These peculiarities include a low total pulsation amplitude, a mode with bizarre amplitudes in the ultraviolet, and a mode harmonic that exceeds the amplitude of its fundamental. Here we present optical time-series spectroscopy of PY Vul acquired with the Keck II Low Resolution Imaging Spectrograph. Our analysis has revealed that the mode with unusual UV amplitudes also has distinguishing characteristics in the optical. Comparison of its line profile variations to models suggests that this mode has a spherical degree of 4. We show that all the other peculiarities in this star are accounted for by a dominant pulsation mode of l = 4 and propose this hypothesis as a solution to the mysteries of PY Vul.