The First Direct Spectroscopic Detection of a White Dwarf Primary in an AM CVn System

We report the results of a synthetic spectral analysis of Hubble STIS spectraof the AM CVn-type cataclysmic variable CP Eri obtained when the system was inquiescence. The FUV spectrum is best fitted by a helium-dominated, hybridcomposition (DBAZ) white dwarf with Teff=17,000K +/- 1000K, log g=8, Heabundance ~1000 times solar, H abundance ~0.1 times solar, metallicity Z ~ 0.05times solar, V sin i = 400 km/s. This is the first directly detected primarywhite dwarf in any AM CVn and the surface abundance and rotation rate for thewhite dwarf primary are the first to be reported for AM CVn systems. Themodel-predicted distance is ~ 1000 pc. The spectral fits using pure Hephotospheres or He-rich accretion disks were significantly less successful.Based upon the analysis of our FUV spectra, CP Eri appears to contain a hybridcomposition DBAZ white dwarf with a metallicity which sets it apart from theother two AM Cvn stars which have been observed in quiescence and aremetal-poor. The implications of this analysis for evolutionary channels leadingto AM CVn systems are discussed.Comment: 13 pages, 3 figure