from the data reduction procedure.

We detail now the effects on these results of possible variations of age and chemical composition within the uncertainties described in Section 3. The mass distribution along the HB (as derived in our analysis) does not depend on the cluster age, but changing the cluster age changes the mass loss necessary to reproduce the HB mass distribution. An increase of age of $1$ Gyr leads to a decrease of the total mass loss along the RGB by $\sim0.02 M_{\odot}$ (because of the decreased evolving mass).\
One can simulate the effect of a variation of \[Fe/H\], \[$\alpha$/Fe\] and/or their sum by considering that at these metallicities scaled solar and $\alpha$-enhanced models (also for the HB) with the same total global metallicity \[M/H\] are coincident (Salaris et al. 1993). We have therefore considered a change $\Delta [M/H]=+0.3$ dex that corresponds to changes of \[Fe/H\] or \[$\alpha$/Fe\] or \[Fe/H\]+\[$\alpha$/Fe\]. We have two effects here. The first one is on the derived mass distribution (at fixed Y) along the HB. For the redder part of the HB we use the ($m_{F336W}$,$m_{F336W}-m_{F555W}$) colour. At fixed colour in the horizontal part of the ($m_{F336W},m_{F336W}-m_{F555W}$) CMD, an increase $\Delta$\[M/H\]=0.3 decreases the derived 