  We present an accurate analysis of the peculiar Horizontal Branch (HB) of the
massive Galactic globular cluster NGC 2808, based on high-resolution far-UV and
optical images of the central region of the cluster obtained with HST. We
confirm the multimodal distribution of stars along the HB: 4 sub-populations
separated by gaps are distinguishable. The detailed comparison with suitable
theoretical models showed that (i) it is not possible to reproduce the
luminosity of the entire HB with a single helium abundance, while an
appropriate modeling is possible for three HB groups by assuming different
helium abundances in the range 0.24 < Y < 0.4 that are consistent with the
multiple populations observed in the Main Sequence; (ii) canonical HB models
are not able to properly match the observational properties of the stars
populating the hottest end of the observed HB distribution, the so called
"blue-hook region". These objects are probably "hot-flashers" , stars that peel
off the red giant branch before reaching the tip and ignite helium at high
effective temperatures. Both of these conclusions are based on the luminosity
of the HB in the optical and UV bands and do not depend on specific assumptions
about mass loss.
