We present the discovery of 15 extremely low mass ( 5 < \log { g } < 7 ) white dwarf candidates , 9 of which are in ultra-compact double-degenerate binaries .
Our targeted ELM Survey sample now includes 76 binaries .
The sample has a lognormal distribution of orbital periods with a median period of 5.4 hr .
The velocity amplitudes imply that the binary companions have a normal distribution of mass with 0.76 M _ { \odot } mean and 0.25 M _ { \odot } dispersion .
Thus extremely low mass white dwarfs are found in binaries with a typical mass ratio of 1:4 .
Statistically speaking , 95 % of the white dwarf binaries have a total mass below the Chandrasekhar mass and thus are not Type Ia supernova progenitors .
Yet half of the observed binaries will merge in less than 6 Gyr due to gravitational wave radiation ; probable outcomes include single massive white dwarfs and stable mass transfer AM CVn binaries .