Ratio of L_UX300 kpc/ (U_XO) plotted against L_opt for all groups. Blue, black, and red colours denote groups in the X-ray overluminous, normal, and underluminous subsamples. Inverted triangles denote X-ray upper limits, where no significant X-ray flux value was detected. We highlight X-ray underluminous systems with properties suggesting an evolved group (called here EXdim groups) with green circles. If these groups are dynamically evolved, their low X-ray emission must have some other cause.