Left panel: dynamical mass-to-light ratios of compact stellar systems plotted vs. their dynamical mass. Data taken from the literature as indicated in the text. Sources are colour coded according to mass. GCs (M < 2 × 10⁶ M_⊙) are blue, low-mass UCDs (2 × 10⁶ M_⊙ < M < 10⁷ M_⊙) are green, high-mass UCDs (M > 10⁷ M_⊙) are red. GC data points are dominated by Local Group measurements, whereas UCD data points are mainly extragalactic (Fornax/Virgo/CenA). Middle panel: dynamical mass-to-light ratios of compact stellar systems plotted vs. their metallicities. Sample and symbols as in the left panel. The dotted lines correspond to predicted stellar M/L ratios for a 13 Gyr stellar population of solar α abundance; top line is from Maraston et al. (2005), bottom line is from Bruzual & Charlot (2003). The solid curve in between is an exponential fit to the mean of the two dotted lines. It represents the maximum stellar M/L for a canonical IMF (Kroupa/Chabrier). Sources with dynamical M/L ratios above that solid line need some extra dark mass, and/or IMF variation. The (magenta) thin solid line below the thick solid line corresponds to the stellar M/L ratio for a younger, 9 Gyr stellar population of solar α abundance. Right panel: dimensionless quantity Ψ = (M/L)_dyn/(M/L)_pop is plotted vs. dynamical mass for the same sources as in previous plots, using Eq. (4) for (M/L)_pop, see also middle panel. The black horizontal line indicates Ψ = 1 for 13 Gyr assumed age (see also black solid line in middle panel), the (magenta) thin solid line indicates Ψ = 1 for 9 Gyr assumed age (magenta solid line in middle panel). We mark UCDs (M > 2 × 10⁶ M_⊙) from the Local Group with (green) filled circles, and massive extragalactic GCs (M < 2 × 10⁶ M_⊙) with (blue) asterisks.