THE UNUSUAL HELIUM VARIABLE AM CANUM-VENATICORUM

The unusual variable star AM CVn has puzzled astronomers for over 40 y ears. This object, both a photometric and spectroscopic variable, is b elieved to contain a pair of hydrogen-deficient white dwarfs of extrem e mass ratio, transferring material via an accretion disk. We examine the photometric properties of AM CVn, analyzing 289 hours of high-spee d photometric data spanning 1976 to 1992. The power spectrum displays significant peaks at 988.7, 1248.8, 1902.5, 2853.8, 3805.2, 4756.5, an d 5707.8 mu Hz (1011.4, 800.8, 525.6, 350.4, 262.8, 210.2, and 175.2 s ). We find no detectable power at 951.3 mu Hz (1051 s), the previously reported main frequency. The 1902.5, 2853.9, and 3805.2 mu Hz peaks a re multiplets, with frequency splitting in each case of 20.77 +/- 0.05 mu Hz. The 1902.5 mu Hz seasonal pulse shapes are identical, within m easurement noise, and maintain the same amplitude and phase as a funct ion of color. We have determined the dominant frequency to be 1902.509 802 +/- 0.00001 mu Hz, with P = +1.71 (+/-0.04) x 10(-11) s s(-1). We discuss the implications of these findings on a model for AM CVn.