The quiescent spectrum of the AM Canum Venaticorum star CP Eridani

We used the 6.5 m Multiple Mirror Telescope to obtain a spectrum of the AM Canum Venaticorum star CP Eridani in quiescence. The spectrum is dominated by He I emission lines, which are clearly double-peaked with a peak-to-peak separation of similar to 1900 km s(-1). The spectrum is similar to that of the longer period AM CVn systems GP Comae Berenices and CE 315, linking the short- and long-period AM CVn systems. In contrast to GP Com and CE 315, the spectrum of CP Eri does not show a central "spike" in the line profiles, but it does show lines of Si ii in emission. The presence of these lines indicates that the material being transferred is of higher metallicity than in GP Com and CE 315, which, combined with the low proper motion of the system, probably excludes a halo origin of the progenitor of CP Eri. We constrain the primary mass to M-1 > 0.27 M-. and the orbital inclination to 33 degrees < i <less than or similar to> 80 degrees. The presence of the He i lines in emission opens up the possibility for phase- resolved spectroscopic studies, which will allow us to determine the system parameters and to study in detail helium accretion disks under highly varying circumstances.