  I present multiband optical and UV Hubble Space Telescope photometry of the
two Galactic globular clusters NGC6388 and NGC6441, in order to investigate the
nature of the physical mechanism(s) responsible for the existence of an
extended blue tail and of a slope in the horizontal branch. Further evidence
that the horizontal branch tilt cannot be interpreted as an effect of
differential reddening is provided, while I show that a possible solution of
the puzzle is to assume that a small fraction of the stellar population in the
two clusters is strongly helium enriched.
