We report on UV and X-ray spectroscopy and broad-band optical observations of the ultraluminous X-ray source in Holmberg II .
Fitting various stellar spectral models to the combined , non-simultaneous data set , we find that normal metallicity stellar spectra are ruled out by the data , while low metallicity , Z = 0.1 Z _ { \odot } , late O-star spectra provide marginally acceptable fits , if we allow for the fact that X-ray ionization from the compact object may reduce or eliminate UV absorption/emission lines from the stellar wind .
By contrast , an irradiated disk model fits both UV and optical data with \chi ^ { 2 } / { dof } = 175.9 / 178 , and matches the nebular extinction with a reddening of E ( \bv ) = 0.05 ^ { +0.05 } _ { -0.04 } .
These results suggest that the UV/optical flux of Holmberg II X-1 may be dominated by X-ray irradiated disk emission .