The ELM Survey. II. Twelve Binary White Dwarf Merger Systems
  We describe new radial velocity and X-ray observations of extremely low-mass
white dwarfs (ELM WDs, ~0.2 Msol) in the Sloan Digital Sky Survey Data Release
4 and the MMT Hypervelocity Star survey. We identify four new short period
binaries, including two merger systems. These observations bring the total
number of short period binary systems identified in our survey to 20. No
main-sequence or neutron star companions are visible in the available optical
photometry, radio, and X-ray data. Thus, the companions are most likely WDs.
Twelve of these systems will merge within a Hubble time due to gravitational
wave radiation. We have now tripled the number of known merging WD systems. We
discuss the characteristics of this merger sample and potential links to
underluminous supernovae, extreme helium stars, AM CVn systems, and other
merger products. We provide new observational tests of the WD mass-period
distribution and cooling models for ELM WDs. We also find evidence for a new
formation channel for single low-mass WDs through binary mergers of two lower
mass objects.
