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 HeII \lambda 4686 emission ( equivalent width of 10 Å ) 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 .