  Characterizing the local space density of double degenerate binary systems is
a complementary approach to broad sky surveys of double degenerates to
determine the expected rates of white dwarf binary mergers, in particular those
that may evolve into other observable phenomena such as extreme helium stars,
Am CVn systems, and supernovae Ia. However, there have been few such systems
detected in local space. We report here the discovery that WD 1242$-$105, a
nearby bright WD, is a double-line spectroscopic binary consisting of two
degenerate DA white dwarfs of similar mass and temperature, despite it
previously having been spectroscopically characterized as a single degenerate.
Follow-up photometry, spectroscopy, and trigonometric parallax have been
obtained in an effort to determine the fundamental parameters of each component
of this system. The binary has a mass ratio of 0.7 and a trigonometric parallax
of 25.5 mas, placing it at a distance of 39 pc. The system's total mass is 0.95
M$_\odot$ and has an orbital period of 2.85 hours, making it the strongest
known gravitational wave source ($\log h = -20.78$) in the mHz regime. Because
of its orbital period and total mass, WD 1242$-$105 is predicted to merge via
gravitational radiation on a timescale of 740 Myr, which will most likely not
result in a catastrophic explosion.
