and are randomly extracted from the entire sample of simulated field stars. This figure immediately reveals that the contamination of field stars is negligible and that the triple sequence is not due field stars.

The $\Delta_{\rm F110W,F160W}$ histogram distribution of stars in five magnitude intervals is plotted in the right panel of Fig. \[fig:pratio\]. To derive each histogram we accounted for stellar completeness and subtracted the field stars. The black continuous lines overimposed on the histograms are the best-fit least-squares three-Gaussian functions, whose components are colored green, magenta and blue. From the average of the population ratio obtained in the five magnitude intervals we obtain that the MS-A, MS-B and MS-C host 26.3$\pm$1.4%, 46.9$\pm$1.3% and 26.8$\pm$2.0%, respectively, of the total number of MS stars. The uncertainties are estimated as the ratio between the r.m.s. of the five population-ratio measurements divided by the square root of four. The fractions of MS-A, MS-B and MS-C stars with respect to the total number of MS stars are estimated from the areas of the corresponding Gaussians. Results are listed in Table 1, where we also provide the dispersion of each Gaussian component. Noticeably, the MS-B is significantly wider than the other two sequences, thus 