Upper panel: surface helium abundance of cooler stars as a function of temperature. The dotted line indicates the solar value (Y = 0.25). Filled circles: this work; empty circles: NGC 6752 (Moni Bidin et al. 2007); empty triangles and squares: NGC 5986 and M 80, respectively (Moni Bidin et al. 2009). Lower panel: trend of surface helium with temperature, obtained binning the data of the upper panel with two different binning schemes (see text for details).