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 .