  The ultraluminous x-ray source (ULX) in the galaxy M82 has been identified as
a possible intermediate-mass black hole. We have found that the x-ray flux from
M82 is modulated with a peak-to-peak amplitude corresponding to an isotropic
luminosity of 2.4x10^40 erg/s in M82 and a period of 62.0 +/- 2.5 days, which
we interpret as the orbital period of the ULX binary. This orbital period
implies that the mass-donor star must be a giant or supergiant. Large
mass-transfer rates, sufficient to fuel the ULX, are expected for a giant-phase
mass donor in an x-ray binary. The giant phase has a short lifetime, indicating
that we see the ULX in M82 in a brief and unusual period of its evolution.
