  We present a model for the prediction of the optical/infra-red emission from
ULXs. In the model, ULXs are binary systems with accretion taking place through
Roche lobe overflow. We show that irradiation effects and presence of an
accretion disk significantly modify the optical/infrared flux compared to
single stars, and also that the system orientation is important. We include
additional constraints from the mass transfer rate to constrain the parameters
of the donor star, and to a lesser extent the mass of the BH. We apply the
model to fit photometric data for several ULX counterparts. We find that most
donor stars are of spectral type B and are older and less massive than reported
elsewhere, but that no late-type donors are admissable. The degeneracy of the
acceptable parameter space will be significantly reduced with observations over
a wider spectral range, and if time-resolved data become available.
