  We present the homogeneous reanalysis of Mg and Al abundances from high
resolution UVES/FLAMES spectra for 31 red giants in the globular cluster NGC
2808. We found a well defined Mg-Al anticorrelation reaching a regime of
subsolar Mg abundance ratios, with a spread of about 1.4 dex in [Al/Fe]. The
main result from the improved statistics of our sample is that the distribution
of stars is not continuous along the anticorrelation as they are neatly
clustered into three distinct clumps each with different chemical composition.
One group (P) shows the primordial composition of field stars of similar
metallicity, and the other two (I and E) have increasing abundances of Al and
decreasing abundances of Mg. The fraction of stars we found in the three
components (P: 68%, I: 19%, E: 13%) is in excellent agreement with the number
ratios computed for the three distinct main sequences in NGC 2808: for the
first time there is a clear correspondence between discrete photometric
sequences of dwarfs and distinct groups of giants with homogeneous chemistry.
The composition of the I group cannot be reproduced by mixing of matter with
extreme processing in hot H-burning and gas with pristine, unprocessed
composition, as also found in the recent analysis of three discrete groups in
NGC 6752. This finding suggests that different classes of polluters were
probably at work also in NGC 2808.
