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The AM Canum Venaticorum (AM CVn) binaries are a rare group ofhydrogen-deficient, ultra-short period, mass-transferring white dwarf binaries,and are possible progenitors of type Ia supernovae. We present time-resolvedspectroscopy of the recently-discovered AM CVn binary SDSS J173047.59+554518.5.The average spectrum shows strong double-peaked helium emission lines, as wellas a variety of metal lines, including neon; this is the second detection ofneon in an AM CVn binary, after the much brighter system GP Com. We detect nocalcium in the accretion disc, a puzzling feature that has been noted in manyof the longer-period AM CVn binaries. We measure an orbital period, from theradial velocities of the emission lines, of 35.2 +/- 0.2 minutes, confirmingthe ultra-compact binary nature of the system. The emission lines seen in SDSSJ1730 are very narrow, although double-peaked, implying a low inclination,face-on accretion disc; using the measured velocities of the line peaks, weestimate i < 11 degrees. This low inclination makes SDSS J1730 an excellentsystem for the identification of emission lines.

The AM CVn binary SDSS J173047.59+554518.5