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We have identified the optical counterpart of the ULX source P13 in the nearby spiral galaxy NGC 7793. The object is a  V  ∼ 20.5 mag star, ten times brighter than any other established counterpart of a ULX in nearby galaxies. Medium resolution optical spectroscopy carried out in 2008 and 2009 with the ESO‐VLT reveals the presence of narrow high order Balmer, He I and Mg II absorption lines indicating a late B‐type supergiant companion star with mass between 10 and 20 M ⊙ . Stellar H β  and He II  λ  4686 Å emission lines are also seen superposed on the photospheric spectrum. We detect different patterns of radial velocity variations from the emission and absorption lines over a time interval of one month. The velocity of the high order Balmer absorption lines changes by ∼100 km s –1  while the H β  and He II  λ  4686 Å emission components vary by about the same amount but with a different phasing. Assuming that the observed velocity changes trace the motion of the mass‐donor star and of the X‐ray source implies a mass of the accreting black hole in the range of 3 to 100 M ⊙  with a most probable value of ∼10 to 20 M ⊙ . We expect an orbital period in the range of 20 to 40 days based on the low density of the supergiant star. P13 is likely in a short‐lived, and thus rare high X‐ray luminosity evolutionary state associated with the ascension of the donor star onto the supergiant stage (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The supergiant optical counterpart of ULX P13 in NGC 7793