  We studied the color distribution of the main sequence of the Globular
Cluster NGC 2808, based on new deep HST-WFPC2 photometry of a field in the
uncrowded outskirts of the cluster. The color distribution of main sequence
stars is wider than expected for a single stellar population, given our
(carefully determined) measurement errors. About 20% of the sample stars are
much bluer than expected and are most plausibly explained as a population
having a much larger helium abundance than the bulk of the main sequence. We
estimate that the helium mass fraction of these stars is Y ~ 0.4. NGC 2808 may
have suffered self-enrichment, with different stellar populations born from the
ejecta of the intermediate mass asymptotic giant branch (AGB) stars of the
first generation. In addition to the Y=0.40 stars, roughly 30% of the stars
should have Y distributed between 0.26-0.29 while 50% have primordial Y, to
explain also the peculiar horizontal branch morphology. Three main stages of
star formation are identified, the first with primordial helium content Y ~
0.24, the second one born from the winds of the most massive AGBs of the first
stellar generation (6-7msun), having Y ~ 0.4, and a third one born from the
matter ejected from less massive AGBs (~ 3.5-4.5msun) with Y ~ 0.26-0.29. For a
long hiatus of time (several 10^7yr) between the second and third generation,
star formation might have been inhibited by the explosion of late Supernovae II
deriving from binary evolution.
