We present the results from a Gemini snapshot radial-velocity survey of 44 low-mass white-dwarf candidates selected from the Sloan Digital Sky Survey (SDSS) spectroscopy. To find sub-hour orbital period binary systems, our time-series spectroscopy had cadences of 2-8min over a period of 20-30min. Through follow-up observations at Gemini and the MMT, we identify four double-degenerate binary systems with periods ranging from 53min to 7h. The shortest period system, SDSS J123549.88+154319.3, was recently identified as a sub-hour period detached binary by Breedt and collaborators. Here, we refine the orbital and physical parameters of this system. High-speed and time-domain survey photometry observations do not reveal eclipses or other photometric effects in any of our targets. We compare the period distribution of these four systems with the orbital period distribution of known double white dwarfs; the median period decreases from 0.64 to 0.24d for M=0.3-0.5M_{sun} to M<0.3M{sun}_ white dwarfs. However, we do not find a statistically significant correlation between the orbital period and white-dwarf mass.

Cone search capability for table J/MNRAS/471/4218/table1 (Physical parameters and the summary of radial velocity measurements for our targets.)

Cone search capability for table J/MNRAS/471/4218/table2 (Orbital parameters)