Nova V5116 Sgr 2005 No .
2 , discovered on 2005 July 4 , was observed with XMM-Newton in March 2007 , 20 months after the optical outburst .
The X-ray spectrum showed that the nova had evolved to a pure supersoft X-ray source , indicative of residual H-burning on top of the white dwarf .
The X-ray light-curve shows abrupt decreases and increases of the flux by a factor 8 with a periodicity of 2.97 h , consistent with the possible orbital period of the system .
The EPIC spectra are well fit with an ONe white dwarf atmosphere model , with the same temperature both in the low and the high flux periods .
This rules out an intrinsic variation of the X-ray source as the origin of the flux changes , and points to a possible partial eclipse as the origin of the variable light curve .
The RGS high resolution spectra support this scenario showing a number of emission features in the low flux state , which either disappear or change into absorption feature in the high flux state .
A new XMM-Newton observation in March 2009 shows the SSS had turned off and V5116 Sgr had evolved into a weaker and harder X-ray source .