  We evaluate the mass of the black holes of GW150914 at their event horizons
via quasi-local energy approach and obtain the values of 71 and 57 solar
masses, compared to their asymptotic values of 36 and 29 units, respectively,
as reported by LIGO. A higher mass at the event horizon is compulsory in order
to overcome the huge negative gravitational potential energy surrounding the
black holes and allow for the emission of gravitational waves during merging.
We estimate the initial mass of the stars which collapsed to form the black
holes from the horizon mass and obtain the impressive values of 95 and 76 solar
masses for these progenitor stars.
