  Here we report a measurement of the orbital period and mass function of GRS
1915+105 which allow us to deduce a mass of 14 +- 4 solar masses for its black
hole. This large mass provides a challenge for black hole formation scenarios
in binaries, since black holes with masses above 5-7 solar masses are hard to
explain. Also, the mass estimate allows us to understand the unique X-ray
variability of GRS 1915+105 as being due to instabilities of a
radiation-pressure dominated disk radiating near the Eddington limit. Finally,
several models are constrained which relate observable X-ray properties to the
spin of black holes in microquasars. Once further calibrated, these relations
may soon turn into a valuable tool to study relativistic effects in strong
gravitational fields.
