The super-soft source XMMU J052016.0-692505 in the LMC

Aims.We report the discovery of the super-soft X-ray source XMMU J052016.0-692505 in the LMC with XMM-Newton. Methods.We analyse the EPIC spectra of XMMU J052016.0-692505 and study the likely optical counterpart LMCV2135. Results.Using an absorbed blackbody spectrum we derive a bolometric luminosity of for the X-ray source at LMC distance. Assuming that the bolometric luminosity does not exceed the Eddington luminosity of a ~ star we derive a blackbody temperature of and an LMC absorbing column density of < . The likely optical counterpart of XMMU J052016.0-692505 is the LMC variable star LMCV2135 which is a MACHO and OGLE variable. The infrared and optical colors and magnitudes of this star are consistent with a hot star of likely spectral type B. The long-term MACHO light curve shows variability with a timescale of 500 and 1000 days. The optical spectra obtained at the 1.9-meter telescope of the South African Astronomical Observatory show strong H and H emission lines (with Å) which indicate a B0-3e star. The radial velocities of the H and H emission lines show a variation from to which is consistent with the systemic velocity of the LMC and an intrinsic variation most likely due to the rotation of the Be disk. We discuss LMCV2135/XMMU J052016.0-692505 as a Be/white dwarf binary system in the LMC. The super-soft X-ray spectrum of the source could be due to a stable nuclear burning white dwarf with a mass of .