of the corresponding object evolving along the HB (${\rm M_{HB}}$) is then given by ${\rm M_{HB} = M_{TRGB} - \Delta M}$ plus a Gaussian random perturbation ${\rm \sigma(\Delta M)}$ (or a uniform probability between ${\rm \Delta M}$ and ${\rm \Delta M}$+${\rm \sigma(\Delta M)}$). The WFPC2 magnitudes of the synthetic star are then determined according to its position along the HB track with appropriate mass and Y– obtained by interpolation among the available set of HB tracks – after an evolutionary time t has been randomly extracted. We determined t assuming that stars reach the ZAHB at a constant rate. We employed a flat probability distribution ranging from zero to ${\rm t_{HB}}$, where ${\rm t_{HB}}$ denotes the time spent from the ZAHB to the He-burning shell ignition along the early AGB. The value of ${\rm t_{HB}}$ is set by the mass with the longest lifetime (the lowest masses for a given Y and Z). This implies that for some synthetic object the randomly selected value of t will be longer than its ${\rm t_{HB}}$ or, in other words, that they have already evolved to the next evolutionary stages. Finally, the derived synthetic magnitudes are perturbed with a Gaussian 1$\sigma$ error determined 