X-ray vs. optical redshift for the 15 sources with spectroscopic or photometric redshift and X-ray redshift satisfying the selection criteria. Red circles (triangles) are for clusters whose photometric (spectroscopic) optical redshift is found in the literature through a match with the NED, while the magenta squares are for clusters whose photometric optical redshift has been computed by us with a dedicated TNG program or with VLT archival image (in one case). Error bars on z_X and z_opt correspond to 1σ.