Distribution of the programme stars in the effective temperature-surface gravity plane. Symbols are as in Fig. 3. We overimpose two α-enhanced isochrones from the BASTI database (Pietrinferni et al. 2006), which were computed for [Fe/H] values appropriate to the two sequences ([Fe/H] =  − 1.22 and  − 1.17) and ages of 10.1 and 10.7 Gyr for b-SGB and f-SGB, respectively. Stellar masses along these isochrones are also given.