  With the aim to constrain multiple populations in the metal-poor globular
cluster NGC6397, we analyse and discuss the chemical compositions of a large
number of elements in 21 red giant branch stars. High-resolution spectra were
obtained with the FLAMES/UVES spectrograph on VLT. We have determined non-LTE
abundances of Na and LTE abundances for the remaining 21 elements, including O,
Mg, Al, alpha, iron-peak, and neutron-capture elements, many of which have not
previously been analysed for this cluster. We have also considered the
influence of possible He enrichment in the analysis of stellar spectra. We find
that the Na abundances of evolved, as well as unevolved, stars show a distinct
bimodality, which suggests the presence of two stellar populations; one
primordial stellar generation with composition similar to field stars, and a
second generation that is enriched in material processed through
hydrogen-burning (enriched in Na and Al and depleted in O and Mg). The cluster
is dominated (75%) by the second generation. The red giant branch show a
similar bimodal distribution in the Stroemgren colour index c_y=c_1-(b-y),
implying a large difference also in N abundance. The two populations have the
same composition of all analysed elements heavier than Al, within the
measurement uncertainty of the analysis, with the possible exception of [Y/Fe].
Using two stars with close to identical stellar parameters, one from each
generation, we estimate the difference in He content, Delta Y=0.01+-0.06, given
the assumption that the mass fraction of iron is the same for the stars.
Finally, we show that winds from fast rotating massive stars of the first
generation can be held responsible for the abundance patterns observed in
NGC6397 second generation long-lived stars and estimate that the initial mass
of the cluster were at least ten times higher than its present-day value.
