We used the 6.5m MMT to obtain a spectrum of the AM CVn star CP Eri in quiescence .
The spectrum is dominated by He i emission lines , which are clearly double peaked with a peak-to-peak separation of \sim 1900 km s ^ { -1 } .
The spectrum is similar to that of the longer period AM CVn systems GP Com and CE 315 , linking the short and the long period AM CVn systems .
In contrast with 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 _ { \odot } and the orbital inclination to 33 \hbox { $ { } ^ { \circ } $ } < i \mathrel { \hbox { \hbox to 0.0 pt { \lower 2.365 pt \hbox { $ \sim% $ } } \kern - 3.0 pt \raise 1.72 pt \hbox { $ < $ } } } 80 \hbox { $ { } ^ { \circ } $ } .
The presence of the He i lines in emission opens up the possibility for phase resolved spectroscopic studies which allows a determination of the system parameters and a detailed study of helium accretion disks under highly varying circumstances .