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Ultra-luminous X-Ray sources are extra-nuclear point sources in externalgalaxies with $L_X=10^{39}$--$10^{41}$ erg/s and are among the most poorlyunderstood X-ray sources. To help understand their nature, we are trying toidentify their optical counterparts by combining images from the Hubble SpaceTelescope and the Chandra Observatory. Here we report upon the opticalcounterpart for the ULX in NGC 5204, which has average X-ray luminosity of$\sim3\times10^{39}$ erg/s and has varied by a factor of 50% over the last 10years. A unique optical counterpart to this ULX is found by carefully comparingthe Chandra ACIS images and HST WFPC2 and ACS/HRC images. The spectral energydistribution and the HST/STIS FUV spectrum of this object show that it is a B0Ib supergiant star with peculiarities, including the $\lambda$1240 N V emissionline that is uncommon in B stellar spectra but has been predicted for X-rayilluminated accretion disks and seen in some X-ray binaries. Study of its FUVspectrum leads to a binary model for this ULX in which the B0 Ib supergiant isoverflowing its Roche Lobe and accreting onto the compact primary, probably ablack hole. This picture predicts an orbital period of $\sim10$ days fordifferent black hole mass, which can be tested by future observations

The Optical Counterpart of an Ultraluminous X‐Ray Source in NGC 5204