Observational evidence for stellar mass binary black holes and their
  coalescence rate
  We review the formation scenarios for binary black holes, and show that their
coalescence rate depends very strongly on the outcome of the second mass
transfer. However, the observations of IC10 X-1, an binary with a massive black
hole accreting from a Wolf-Rayet star proves that this mass transfer can be
stable. We analyze the future evolution of IC10 X-1 and show that it is very
likely to form a binary black hole system merging in a few Gyrs. We estimate
the coalescence rate density of such systems to be $ 0.06 {\rm Mpc}^{-3} {\rm
Myr}^{-1}$, and the detection rate for the current LIGO/VIRGO of $ 0.69 {\rm
yr}^{-1} $, a much higher value than the expected double neutron star rate.
Thus the first detection of a gravitational wave source is likely to be a
coalescence of a binary black hole.
