The nature of high Lx/Lopt sources

GT While the majority of Xray sources from serendipitous surveys have beenidentified, a significant fraction (1020%) have no optical counterpart down tomagnitudes, Rvirgul20. These objects have eluded identification to date; possiblecandidates are diskless quasars, isolated neutron stars, opticallypoorclusters, or even a new category of high Lx/Lopt sources not yet discovered. Wepropose observations of two bright Xray sources from WGACAT with high opticalto Xray flux ratios (>80, this limit is higher than that for any known classof object). The XMM images will give accurate Xray positions for these sourceswhich, together with the OM data, provide firm evidence for theiridentification or for followup work, if necessary. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]