FORS2 R-band pre-images of HCG 22 (left) and HCG 90 (right), of dimension 7 × 7′, corresponding to about 65 × 65   kpc at the distance of the groups. Blue diamonds show the spectroscopic targets and those marked with red circles are confirmed compact members of the HCGs (most in the mass range of UCDs), i.e. those with radial velocities within the group range (cf. Fig. 4). Note the difference in their spatial distribution: UCD candidates in HCG 22 are clustered around the central galaxy, while the few UCDs in HCG 90 have more dispersed distribution. Overlaid are also the HST pointings used in our analysis (WFPC2 for HCG 22 and ACS/WFC for HCG 90). Labels in HCG 22 indicate the name of the target at which the WF3 chip was centred. Physical scale and orientation are indicated in the lower left corner and are the same for both fields. A colour version of this figure, showing a high-resolution HST zoom at the dusty disk in the central 5   kpc of NGC 1199 and gas dominated spiral in HCG 90, is available as an online material.