  To check the impact of the multiple population scenario for globular clusters
on their HB, we present an analysis of the composition of 110 red HB (RHB)
stars in 47 Tucanae and of 61 blue HB (BHB) and 30 RHB stars in M5. In 47 Tuc
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 (Delta Y<0.03) in 47 Tuc to reproduce our observations; this agrees with
previous analyses. There is possibly a correlation between the abundances of p-
and n-capture elements in 47 Tuc. If confirmed, this might suggest that AGB
stars of moderate mass contributed to the gas from which second-generation
stars formed. Considering the selection effects in our sample (which does not
include stars warmer than 11000 K and RR Lyrae variables) is important to
understand our results for M5. In this case, we find that, as expected, RHB
stars are Na-poor and O-rich, and likely belong to the primordial population.
There is a clear correlation of the [Na/O] ratio and N abundance with colour
along the BHB. A derivation of the He abundance for these stars yields a low
value of Y=0.22\pm 0.03. This is expected because HB stars of a putative
He-rich population in this cluster should be warmer than 11000 K, and would
accordingly not have been sampled by our analysis. However, we need some
additional source of scatter in the total mass loss of stars climbing up the
RGB to reproduce our results for M5. Finally, we found a C-star on the HB of 47
Tuc and a Ba-rich, fast-rotating, likely binary star on the HB of M5. These
stars are among the brightest and coolest HB stars.
