Lick line index diagnostic plots for a sub-sample of the compact group members with high enough S/N (error in  ⟨ Fe ⟩  index below 0.5 dex). Left panel: diagnostic plot for alpha abundances, plotting  ⟨ Fe ⟩  vs. Mgb index. We show lines of identical alpha abundances for both age of 12 Gyr (solid line) and 4 Gyr (dashed line), using the model predictions of Thomas et al. (2003). The two most metal-rich members of either group appear to have super-solar alpha abundances. Right panel: diagnostic plot for age estimation, plotting H_β vs.  ⟨ Fe ⟩  index for four different ages 12, 7, 3, and 1 Gyr from bottom to top (solar α-abundance). The three UCDs in HCG 90 appear consistent with very old ages, as do the two HCG 22 UCDs HCG22_UCD001 and HCG22_UCD005 (see Tables 1 and 2). Numbers on the symbols are the ID of each object, where the prefixes were omitted for clarity.