Left column: I vs. B – I CMDs showing stars with Giraffe HR (top) or MR (bottom) spectra. Red squares and blue diamonds show the old and intermediate-age stellar components. Dotted lines mark the boundaries of the gravity bins adopted for the spectrum-stacking procedure. Two isochrones from the BaSTI database (Pietrinferni et al. 2004, 2006) are shown as red dashed and blue dot-dashed lines. The adopted distance modulus and reddening are indicated (Coppola et al. 2013). Right column: colored symbols show the positions of the stacked spectra in the stellar parameter log g vs. T_eff plane.