  We study a complete, colour-selected sample of double-degenerate binary
systems containing extremely low mass (ELM) <0.25 Msol white dwarfs (WDs). We
show, for the first time, that Milky Way disk ELM WDs have a merger rate of
approximately 4 x 10^(-5)/yr due to gravitational wave radiation. The merger
end-product depends on the mass ratio of the binary. The ELM WD systems that
undergo stable mass transfer can account for >3% of AM CVn stars. More
importantly, the ELM WD systems that may detonate merge at a rate comparable to
the estimated rate of underluminous SNe, rare explosions estimated to produce
only ~0.2 Msol worth of ejecta. At least 25% of our ELM WD sample belong to the
old thick disk and halo components of the Milky Way. Thus, if merging ELM WD
systems are the progenitors of underluminous SNe, transient surveys must find
them in both elliptical and spiral galaxies.
