V5116 Sagittarii, an Eclipsing Supersoft Postoutburst Nova?

V5116 Sgr (Nova Sgr 2005 No. 2), discovered on 2005 July 4, was observed with XMM-Newton in 2007 March, 20 months after the optical outburst. The X-ray spectrum shows that the nova had evolved to a pure supersoft X-ray source, with no significant emission at energies above 1 keV. The X-ray light curve shows abrupt decreases and increases of the flux by a factor ~8. It is consistent with a periodicity of 2.97 hr, the orbital period suggested by Dobrotka and coworkers, although the observation lasted just a little more than a whole period. We estimate the distance to V5116 Sgr to be 11 ± 3 kpc. A simple blackbody model does not fit correctly the EPIC spectra, with χ²_ν > 4. In contrast, ONe-rich white dwarf atmosphere models provide a good fit, with N_H = (1.3 +/- 0.1) × 10²¹ cm^-2, T = (6.1 +/- 0.1) × 10⁵ K, and L = (3.9 +/- 0.8) × 10³⁷(D/10 kpc)² ergs s^-1 (during the high-flux periods). This is consistent with residual hydrogen burning in the white dwarf envelope. The white dwarf atmosphere temperature is the same both in the low- and the high-flux periods, ruling out an intrinsic variation of the X-ray source as the origin of the flux changes. We speculate that the X-ray light curve may result from a partial coverage by an asymmetric accretion disk in a high-inclination system.