Discovery of a Detached, Eclipsing 40 min Period Double White Dwarf
  Binary and a Friend: Implications for He+CO White Dwarf Mergers
  We report the discovery of two detached double white dwarf (WD) binaries,
SDSS J082239.546+304857.19 and SDSS J104336.275+055149.90, with orbital periods
of 40 and 46 min, respectively. The 40 min system is eclipsing; it is composed
of a 0.30 Msun and a 0.52 Msun WD. The 46 min system is a likely LISA
verification binary. The short 20 Myr and ~34 Myr gravitational wave merger
times of the two binaries imply that many more such systems have formed and
merged over the age of the Milky Way. We update the estimated Milky Way He+CO
WD binary merger rate and affirm our previously published result: He+CO WD
binaries merge at a rate at least 40 times greater than the formation rate of
stable mass-transfer AM~CVn binaries, and so the majority must have unstable
mass-transfer. The implication is that spin-orbit coupling in He+CO WD mergers
is weak, or perhaps nova-like outbursts drive He+CO WDs into merger as proposed
by Shen.
