Comparison between C₂ index values and the models of Maraston (2005). The ages of the clusters are the mean ones from Table 1 with error bars reflecting the rms. For guidance, in the top panel we reproduce the K-band D_CO index from Paper I (their Fig. 9). In both panels with black arrows we indicated how the spectrum of NGC 2173 changes when adding different fractions of LMC carbon star light to it. The red arrows indicate the changes in the same spectrum, but adding different fractions of the averaged Milky Way carbon star spectrum of Lançon & Mouhcine (2002). See text for a detailed description.