We present the results of the deepest optically identified X-ray survey yet made. The X-ray survey was made with the ROSAT PSPC and reaches a flux limit of 1.6 ×10 −15 erg cm −2 s −1 (0.5-2.0 keV). Above a flux limit of 2 × 10 −15 erg cm −2 s −1 we define a complete sample of 70 sources of which 59 are identified. For a further 5 sources we have tentative identifications and for a further 4 the X-ray error-boxes are blank to R=23 mag. At brighter fluxes ( ≥ 10 −14 erg cm −2 s −1) we confirm the results of previous less deep X-ray surveys with 84 % of our sources being QSOs. However at the faint flux limit the survey is dominated by a population of galaxies with narrow emission lines (NELGs). In addition at intermediate fluxes we find a small number of groups and clusters of galaxies at redshifts generally> 0.3. Most of these groups are poor systems of low X-ray luminosity and the number which we find is consistent with a zero evolutionary scenario, unlike the situation for high luminosity clusters a
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