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Globular clusters: individual (M3, M13, M79); stars: evolution – Horizontal Branch; ultraviolet: stars

Introduction
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Horizontal branch (HB) stars are the progeny of low-mass Red Giant Branch stars (RGB) burning helium in their cores and hydrogen in a shell around it (Hoyle & Schwarzschild 1955). As first noticed by Iben & Rood (1970), the different HB star colour distributions observed in old stellar systems, is the reflection of the amount of mass lost during the RGB phase.\
The scientific community agrees from nearly fifty years about the fact that the principal parameter governing the shape of HBs in Galactic Globular Clusters (GGCs) is metallicity. The general rule is that metal-rich systems have red HBs, while in the metal-poor ones stars are distributed on average at higher effective temperatures (bluer colours). Several exceptions have come out during the last decades; remarkable cases the cases of NGC6388 and NGC6441 (Rich et al. 1997), which despite their metallicity ($[Fe/H]\sim-0.6$) show some of the bluest HBs known among GGCs (Busso et al. 2007; Dalessandro et al. 2008). Moreover several clusters, sharing similar metal content, reveal different HB morphologies, typical cases being the pairs NGC5927 - NGC6388 at high metallicities ($[Fe/H]\sim-0.4$), M3 - 