A CENSUS OF AM CVn STARS: THREE NEW CANDIDATES AND ONE CONFIRMED 48.3-MINUTE BINARY

We present three new candidate AM CVn binaries, and one confirmed new system, from a spectroscopic survey of color-selected objects from the Sloan Digital Sky Survey (SDSS). All four systems were found from their helium emission lines in low-resolution spectra taken on the Hale telescope at Palomar, the Nordic Optical Telescope, and the William Herschel Telescope on La Palma. The ultra-compact binary nature of SDSS J090221.35+381941.9 was confirmed using phase-resolved spectroscopy at the Keck-I telescope. From the characteristic radial velocity "S-wave" observed in the helium emission lines, we measure an orbital period of 48.31 +/- 0.08 minutes. The continuum emission can be described with a blackbody or a helium white dwarf atmosphere of T-eff similar to 15,000 K, in agreement with theoretical cooling models for relatively massive accretors and/or donors. The absence in the spectrum of broad helium absorption lines from the accreting white dwarf suggests that the accreting white dwarf cannot be much hotter than 15,000 K, or that an additional component such as the accretion disk contributes substantially to the optical flux. Two of the candidate systems, SDSS J152509.57+360054.5 and SDSS J172102.48+273301.2, do show helium absorption in the blue part of their spectra in addition to the characteristic helium emission lines. This in combination with the high effective temperatures of similar to 18,000 K and similar to 16,000 K suggests both to be at orbital periods below similar to 40 minutes. The third candidate, SDSS J164228.06+193410.0, exhibits remarkably strong helium emission on top of a relatively cool (T-eff similar to 12,000 K) continuum, indicating an orbital period above similar to 50 minutes.