WHOLE EARTH TELESCOPE OBSERVATIONS OF AM CANUM-VENATICORUM - DISCOSEISMOLOGY AT LAST

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 Telescop e (WET) project.(1) This star is believed to be an ultra-short period cataclysmic binary. In the temporal spectrum of the light curve we fin d a series of 5 harmonically related frequency mo dulations, some with sidebands with a constant frequency spacing of 20.8 mu Hz always on t he high-frequency side. The set of harmonics has a fundamental frequen cy of 951 mu Hz. NO modulation is detected at this frequency in the li ght curve. In addition, modulations with frequencies 972.5 and 988.9 m u Hz are detected with low amplitudes. The structure of the dominant 1 903 mu Hz modulation explains part of the ''phase jitter'' observed ea rlier. The amplitude of this peak is modulated with a period of 13.32 +/- 0.05 hrs. The same period is detected in absorption line shape mod ulations, most likely arising from variable aspects of the outer parts of the disk (Patterson et al. 1993). The observed periodic light modu lations can be explained as a combination of aspect variations of disk modifications due to tidally induced shocks as described by Savonie e t al. (1994), which leads to a two-armed spiral structure, and the mod e coupling model of Lubow (1991), which leads to a three-fold azimutha l symmetry in the outer parts of the disk and a prograde precessing wa ve. The two-and three-fold aszimuthal structures are stationary in the binary frame and explain the higher harmonics of the orbital period w e observe in the light curve. This may be the first example of a succe ssful disco-seismological interpretation. In addition we propose that the variable amplitude modulation at 989 mu Hz may be explained as a g -mode pulsation, which indicates that the central white dwarf may be a DO with a hot envelope.