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 spectra of the AM CVn-type cataclysmic variable CP Eri obtained when the system was in quiescence. The FUV spectrum is best fitted by a helium-dominated, hybrid composition (DBAZ) white dwarf with Teff ∼ 17, 000K ± 1000K, log g∼ 8, He abundance ∼ 1000× solar, H abundance ∼ 0.1× solar, metallicity Z ∼ 0.05× solar, V sin i ∼ 400 km/s±100 km/s. This is the first directly detected primary white dwarf in any AM CVn and the surface abundance and rotation rate for the white dwarf primary are the first to be reported for AM CVn systems. The modelpredicted distance is ∼ 1000 pc. The spectral fits using pure He photospheres or He-rich accretion disks were significantly less successful. Based upon the analysis of our FUV spectra, CP Eri appears to contain a hybrid composition DBAZ white dwarf with a metallicity which sets it apart from the other two AM Cvn stars which have been observed in quiescence and are metal-poor. The implications of this analysis for evolutionary channels leading to AM CVn systems are discussed.