spectra are generated with SYNTHE (Kurucz & Avrett 1981) over the wavelength range between 8,500 and 19,000 $\AA$ and by using model atmospheres calculated by using the ATLAS12 code, which was developed by Robert Kurucz (e.g.Kurucz 1970, 1993, 2005; Castelli 2005) and ported to Linux by Sbordone et al.(2004, 2007). We included molecular line lists for CO, C$_{2}$, CN, OH, MgH, SiH, H$_{2}$O, TiO, VO, ZrO from Partridge & Schwenke(1997), Schwenke (1998). Each synthetic spectrum has been convolved with the transmission curves of the F110W and F160W WFC3/NIR filters to derive the corresponding fluxes. Finally, we calculated the difference between the F110W and F160W magnitudes derived from each comparison spectrum and the corresponding reference spectrum ($\delta$m). The blue, green and red isochrones plotted in the right panel of Fig. \[fig:iso\] have been derived by adding $\delta$m to the corresponding isochrones plotted in the left panel.

The isochrones that account for the abundances of He, C, N, O of the distinct populations qualitatively reproduce the observations and show that the blue, middle, and red MSs correspond to the populations A, B and C defined by Milone et al.(2013). As expected, the isochrones of populations A and C are qualitatively similar to 