Colour − magnitude diagrams of HCG compact members (left panel) and all preselected targets (right panel). Left panel: (B − R)₀ is plotted vs. M_R for spectroscopically confirmed members in the two groups. The dotted lines are model predictions for simple stellar populations from Bruzual & Charlot (2003) for a range of metallicities and ages, and a stellar mass of 2 × 10⁶   M_⊙, which marks the limit between normal GCs and UCDs (e.g. Mieske et al. 2008). From left to right the lines correspond to metallicities [Z/H]  = −1.6, −0.6, 0.1, and 0.4 dex. The open squares indicate ages of 2, 5, and 12 Gyr,  from top to bottom. The solid (blue) line indicates the 12 Gyr isochrone for a stellar mass of 2 × 10⁶   M_⊙. Right panel: analogous to the left panel, but for a wider colour − magnitude range to show the confirmed group members and the preselected targets of our survey. Filled red circles and black squares indicate the confirmed group members, the same as in the left panel. Open circles indicate all other objects that were included in the MXU masks. Sources marked by asterisks are those which were high priority candidates, according to their colour − magnitude values and morphology. Small dots indicate target sources of lower priority (colours fall outside the selection color range, or of more resolved morphology).