anti-correlation (Sneden et al. 2004, Carretta et al. 2009), comparable to the case of NGC2808, but photometric analyses currently available do not show evidences for a multimodal MS. Here we analyze our HB photometry with – as stated before – the main aim of constraining the range of initial He values of the cluster stars. The use of the F160BW filter – not considered in the investigations by Caloi & D’Antona (2005) and D’Antona & Caloi (2008) – will set strong constraints on the maximum value of Y.

The first step of our analysis consists in defining the highest possible Y value necessary to fit the observed HB. To this purpose we calculated a synthetic HB with this preliminary set of parameters (uniform distributions for Y and Gaussian for M ): ${\rm Y_{min}}$=0.246, ${\rm \Delta Y}$=0.104, ${\rm \Delta M}$=0.21${\rm M_{\odot}}$, ${\rm \sigma(\Delta M)}$=0.025${\rm M_{\odot}}$. Even though this selection of parameters is not optimized for a perfect fit to the observed stellar distribution along the HB, it will suffice to put a first general constraint to the maximum possible range of initial Y. The number of stars in this simulation is higher than the observed sample, to reduce the effect of 