  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
spectroscopy. To find sub-hour orbital period binary systems, our time-series
spectroscopy had cadences of 2 to 8 min over a period of 20-30 min. Through
follow-up observations at Gemini and the MMT, we identify four double
degenerate binary systems with periods ranging from 53 min to 7 h. The shortest
period system, SDSS J123549.88+154319.3, was recently identified as a subhour
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 0.24 d for $M=0.3-0.5 M_{\odot}$ to $M<0.3
M_{\odot}$ white dwarfs. However, we do not find a statistically significant
correlation between the orbital period and white dwarf mass.
