1998A&A...332..939S

We report the results of 143.2 hours of time-series photometry over a 12 day period for AM CVn (= HZ 29) as part of the Whole Earth Telescope (WET) project. This star is believed to be an ultra-short period cataclysmic binary. In the temporal spectrum of the light curve we find a series of 5 harmonically related frequency mo dulations, some with sidebands with a constant frequency spacing of 20.8µHz always on the high-frequency side. The set of harmonics has a funda- mental frequency of 951µHz. No modulation is detected at this frequency in the light curve. In addition, modulations with frequencies 972.5 and 988.9µHz are detected with low amplitudes. The structure of the dominant 1903µHz modulation explains part of the ``phase jitter'' observed earlier. The amplitude of this peak is modulated with a period of 13.32±0.05hrs. The same period is detected in absorption line shape modulations, most likely arising from variable aspects of the outer parts of the disk (Patterson et al. 1993). The observed periodic light modulations can be explained as a combination of aspect variations of disk modifications due to tidally induced shocks as described by Savonie et al. (1994), which leads to a two-armed spiral structure, and the mode coupling model of Lubow (1991), which leads to a three-fold azimuthal symmetry in the outer parts of the disk and a prograde precessing wave. The two- and three-fold aszimuthal structures are stationary in the binary frame and explain the higher harmonics of the orbital period we observe in the light curve. This may be the first example of a successful disco- seismological interpretation. In addition we propose that the variable amplitude modu-lation at 989µHz may be explained as a g-mode pulsation, which indicates that the central white dwarf may be a DO with a hot envelope.