Difference between the Hess diagrams of the full photometry (see Figure 2) and the outer part (r>15′) photometry (see Figure 7). The blue line is the same best fitting isochrone shown in Figure 2, while the red curved is the BASTI isochrone that bast matches the Padova isochrone. Cyan curve is the He-enhanced, [M/H]-poor BASTI isochrone that best reproduce the locus of SG stars. Parameters for the two BASTI isochrones are indicated.