  On June 8, 2017 at 02:01:16.49 UTC, a gravitational-wave signal from the
merger of two stellar-mass black holes was observed by the two Advanced LIGO
detectors with a network signal-to-noise ratio of 13. This system is the
lightest black hole binary so far observed, with component masses
$12^{+7}_{-2}\,M_\odot$ and $7^{+2}_{-2}\,M_\odot$ (90% credible intervals).
These lie in the range of measured black hole masses in low-mass X-ray
binaries, thus allowing us to compare black holes detected through
gravitational waves with electromagnetic observations. The source's luminosity
distance is $340^{+140}_{-140}$ Mpc, corresponding to redshift
$0.07^{+0.03}_{-0.03}$. We verify that the signal waveform is consistent with
the predictions of general relativity.
