  We present optical observations of an ultraluminous X-ray source (ULX) in
Holmberg IX, a dwarf galaxy near M81. The ULX has an average X-ray luminosity
of some 10^{40} erg/s. It is located in a huge (400pc x 300pc) ionized nebula
being much larger than normal supernova remnants. From the observed emission
lines (widths and ratios) we find that the structure is due to collisional
excitation by shocks, rather than by photoionization.
  We identify the optical counterpart to be a 22.8 mag blue star (M_V=-5.0)
belonging to a small stellar cluster. From isochrone fitting of our
multi-colour photometry we determine a cluster age of 20 to 50 Myr. We also
discovered strong stellar HeII4686 emission (equivalent width of 10 A) which
proves the identification with the X-ray source, and which suggests the
presence of an X-ray heated accretion disc around the putative black hole.
