The ELM Survey. III. A Successful Targeted Survey for Extremely Low Mass
  White Dwarfs
  Extremely low mass (ELM) white dwarfs (WDs) with masses <0.25 Msun are rare
objects that result from compact binary evolution. Here, we present a targeted
spectroscopic survey of ELM WD candidates selected by color. The survey is 71%
complete and has uncovered 18 new ELM WDs. Of the 7 ELM WDs with follow-up
observations, 6 are short-period binaries and 4 have merger times less than 5
Gyr. The most intriguing object, J1741+6526, likely has either a pulsar
companion or a massive WD companion making the system a possible supernova Type
Ia or .Ia progenitor. The overall ELM Survey has now identified 19 double
degenerate binaries with <10 Gyr merger times. The significant absence of short
orbital period ELM WDs at cool temperatures suggests that common envelope
evolution creates ELM WDs directly in short period systems. At least one-third
of the merging systems are halo objects, thus ELM WD binaries continue to form
and merge in both the disk and the halo.
