  Ultraluminous X-Ray Objects are off-nucleus point sources with
$L_X=10^{39}$-$10^{41}$ erg/s and are among the most poorly understood X-ray
sources. To help understand their nature, we are trying to identify their
optical counterparts by combining images from the Hubble Space Telescope and
the Chandra Observatory. Here we report an optical counterpart for ULX NGC 3031
X-11, which has average X-ray luminosity of $\sim2\times10^{39}$ erg/s and has
varied by a factor of 40% over the last 20 years. We find a unique optical
counterpart with the magnitude and color of an O8V star and we identify this as
the secondary in a binary system. The primary is believed to be a black hole of
approximately 18 $M_\odot$, based on analyses of ACIS and ASCA spectra. This
binary system probably is powered by mass transfer from the O8V star onto the
black hole, with the Roche Lobe equal to the stellar radius. This model
predicts an orbital period of $\sim1.8$ days, which can be tested by future
observations.
