---
abstract: |
    We present a detailed comparison between far-UV/optical colour Magnitude Diagrams obtained with high-resolution [*Hubble Space Telescope*]{} data and suitable theoretical models for three Galactic Globular Clusters: M3, M13 and M79.\
    These systems represents a [“classical”]{} example of clusters in the intermediate metallicity regime that, even sharing similar metal content and age, show remarkably different Horizontal Branch morphologies. As a consequence, the observed differences in the colour distributions of Horizontal Branch stars cannot be interpreted in terms of either first (metallicity) or a second parameter such as age.\
    We investigate here the possible role of variations of initial Helium abundance ($Y$). Thanks to the use of a proper setup of far-UV filters, we are able to put strong constraints on the maximum Y ($Y_{max}$) values compatible with the data. We find differences $\Delta Y_{max}\sim 0.02-0.04$ between the clusters with M13 showing the largest value ($Y_{max}\sim0.30$) and M3 the smallest ($Y_{max}\sim0.27$). In general we observe that these values are correlated with the colour extensions of their Horizontal Branches and with the range of the observed Na-O anti-correlations.
date: '18 December, 2012'
title: 'The Horizontal Branch in the UV colour Magnitude Diagrams. II. The case of M3, M13 and M79.'
