I present multiband optical and UV Hubble Space Telescope photometry of the two Galactic globular clusters NGC 6388 and NGC 6441 , 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 can not 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 ( Y \sim 0.40 in NGC 6388 and Y \sim 0.35 in NGC 6441 ) .