  We present deep K-band imaging at the positions of four very faint X-ray
sources found in the UK ROSAT Deep Survey to have no optical counterpart
brighter than R~23. Likely identifications are found within the ROSAT error
circle in all four fields with R-K colours of between 3.2 +/- 0.4 and 6.4 +/-
0.6. From a consideration of the R-K colours and X-ray to optical luminosity
ratios of the candidate identifications, we tentatively classify two of the
X-ray sources as very distant (z ~ 1) clusters of galaxies, one as a narrow
emission line galaxy and one as an obscured QSO.
