Program: GS-2016B-Q-48

A new discovery space for short period binary white dwarfs has been opened up with the availability of SDSS Data Release 10 spectroscopy. We recently fit all of the SDSS DR10 stellar spectra to identify relatively hot ELM white dwarfs with M < 0.35 Msun. Based on our previous observations, all of these objects should be in binary systems with P<1 day orbits, and 43% should be in P<0.1 d systems. We were awarded Gemini time in 2016A to follow-up 33 targets. This program is still ongoing, and we identified a 53 min period binary in Feb 2016. Here we propose to obtain follow-up Gemini spectroscopic observations of the remaining 10 fall targets. Our two major science goals are to discover detached gravitational wave sources for fundamental tests of general relativity, and to constrain the formation rate and space density of merging white dwarfs. The latter is important for constraining the contribution of double degenerates to Type Ia and underluminous supernovae.