The ELM Survey. IV. 24 White Dwarf Merger Systems
  We present new radial velocity and X-ray observations of extremely low-mass
(ELM, 0.2 Msol) white dwarf candidates in the Sloan Digital Sky Survey (SDSS)
Data Release 7 area. We identify seven new binary systems with 1-18 h orbital
periods. Five of the systems will merge due to gravitational wave radiation
within 10 Gyr, bringing the total number of merger systems found in the ELM
Survey to 24. The ELM Survey has now quintupled the known merger white dwarf
population. It has also discovered the eight shortest period detached binary
white dwarf systems currently known. We discuss the characteristics of the
merger and non-merger systems observed in the ELM Survey, including their
future evolution. About half of the systems have extreme mass ratios. These are
the progenitors of the AM Canum Venaticorum systems and supernovae .Ia. The
remaining targets will lead to the formation of extreme helium stars,
subdwarfs, or massive white dwarfs. We identify three targets that are
excellent gravitational wave sources. These should be detected by the Laser
Interferometer Space Antenna (LISA)-like missions within the first year of
operation. The remaining targets are important indicators of what the Galactic
foreground may look like for gravitational wave observatories.
