Optical counterpart of the ultraluminous X-ray source NGC 1313 X-2

We present new optical and Chandra observations of the field containing the ultraluminous X-ray source NGC1313 X-2. On an ESO 3.6 m image, the Chandra error box embraces a R=21.6 mag stellar-like object and excludes a previously proposed optical counterpart. The resulting X-ray/optical flux ratio of NGC 1313 X-2 is ~500. The value of fX/fopt, the X-ray variability history and spectral distribution indicate a luminous X-ray binary in NGC 1313 as a likely explanation for NGC 1313 X-2. The inferred optical luminosity (L~10^5 Lsolar) is consistent with that of a ~10 Msolar companion.