The Na-O anticorrelation in horizontal branch stars : III. 47 Tucanae and M 5

To check the impact of the multiple population scenario for globular clusters on their horizontal branch (HB), we present an analysis of the composition of 110 red HB (RHB) stars in 47Tucanae and of 61 blue HB (BHB) and 30 RHB stars in M5. In 47Tuc we found tight relations between the colours of the stars and their abundances of p-capture elements. This strongly supports the idea that the He content - which is expected to be closely correlated with the abundances of p-capture elements - is the third parameter (after overall metallicity and age) that determines the colour of HB stars. However, the range in He abundance must be small (ΔY