VLT Observations of the Ultraluminous X-Ray Source NGC 1313 X-2

We present archive ESO VLT photometric and spectroscopic data of theUltraluminous X-ray source NGC 1313 X-2. The superb quality of the VLT imagesreveals that two distinct objects, with R magnitudes 23.7 and 23.6, are visibleinside the Chandra error box. The two objects, separated by 0.75 arcsec, wereunresolved in our previous ESO 3.6 m+EFOSC image. We show that both are starsin NGC 1313, the first a B0-O9 main sequence star of ~20 Msun, while the seconda G supergiant of ~10 Msun. Irrespectively of which of the two objects theactual counterpart is, this implies that NGC 1313 X-2 is a high mass X-raybinary with a very massive donor.Comment: 7 pages, 4 figures, accepted for publication in ApJ Letter