  When a bright spot in a high-inclination disk orbits a black hole, its light
is periodically lensed and Doppler shifted. If the spots occupy a narrow range
of radii, a quasi-periodic oscillation (QPO) will result. I calculate the QPO
frequency assuming that the spots appear near r_max - the radius of the maximal
proper radiation flux from the disk. The calculated frequency depends on the
black hole mass and spin. For the microquasar GRO J1655-40, the black hole mass
is known and a 300 Hz QPO was observed. The inferred black hole spin is about
60% of the maximal. The orbit precession frequency at r_max is 20Hz, this QPO
was also observed in GRO J1655-40. An approximate formula for r_max is given.
