F. Grisé, M. W. Pakull, R. Soria, C. Motch,
I. A. Smith, S. D. Ryder, & M. Böttcher
Astronomy & Astrophysics, 486, 151 (2008)
NGC 1313 X-2 is one of the brightest ultraluminous X-ray sources in the sky, both at X-ray and optical wavelengths. Therefore quite a few studies of available VLT and HST data have appeared in the literature. Here, we present our analysis of VLT/FORS1 and HST/ACS photometric data, confirming the identification of the optical counterpart. We also show that the counterpart is part of a poor cluster with an age of 20 Myr leading to an upper mass limit of some 12 M0 for the mass donor. We attribute the difference with previous studies to isochrones in the V and B band HST/ACS photometric system which appears to be incompatible with the ones in the Johnson-Cousins system. The counterpart exhibits significant photometric variability of about 0.2 mag amplitude, both between the two HST observations and during the one month monitoring with the VLT. This includes variability within one night and suggests that the light is dominated by the accretion in the system and not by the mass donor.