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NGC 1313 X-2 is one of the brightest ultraluminous X-ray sources in the sky,at both X-ray and optical wavelengths; therefore, quite a few studies ofavailable ESO VLT and HST data have appeared in the literature. Here, wepresent our analysis of VLT/FORS1 and HST/ACS photometric data, confirming theidentification of the B ~ 23 mag blue optical counterpart. We show that thesystem is part of a poor cluster with an age of 20 Myr, leading to an uppermass limit of some 12 M_sun for the mass donor. We attribute the differentresults with respect to earlier studies to the use of isochrones in the F435Wand F555W HST/ACS photometric system that appear to be incompatible with thecorresponding Johnson B and V isochrones. The counterpart exhibits significantphotometric variability of about 0.2 mag amplitude, both between the two HSTobservations and during the one month of monitoring with the VLT. This includesvariability within one night and suggests that the light is dominated by theaccretion disk in the system and not by the mass donor.

The ultraluminous X-ray source NGC 1313 X-2 - Its optical counterpart and environment