Leaving aside for a moment the open problem of the RGB mass loss, the distribution of Y among stars fed onto the HB will depend on the convolution of the unknown (at least for these three clusters) initial Y distribution among MS stars, with the Y-dependent evolutionary timescales along MS and RGB predicted by stellar models. As discussed before, in absence of constraints from the MS, we assumed here as a first order approximation either Gaussian or uniform probability distribution for Y among HB stars. As a consequence, the derived distribution of Y and mass range (at fixed Y) are most probably just a ’reasonable’ approximation to the real ones. A much more solid result is the total range of Y covered by the stars in each individual cluster, because – thanks to our UV CMDs – this is strongly constrained by the observed stellar magnitudes $m_{F160BW}$ (when available).

![Observed (left panel) vs synthetic (right panel) ($m_{F336W},m_{F336W}-m_{F555W}$) CMD of M13 HB. ZAHB sequences for Y=0.246 (solid line), 0.300 (dashed line) and 0.350 (dot-dashed line) are also displayed. The rectangular box marks the RHB region (see text for details).[]{data-label="synthM13_1"}](f5.ps){width="84mm"}

M13
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Spectroscopic analyses of this cluster reveal a well developed Na-O 