Context : The chemical compositions of globular clusters provide important information on the star formation that occurred at very early times in the Galaxy .
In particular the abundance patterns of elements with atomic number z \leq 13 may shed light on the properties of stars that early on enriched parts of the star-forming gas with the rest-products of hydrogen-burning at high temperatures .

Aims : We analyse and discuss the chemical compositions of a large number of elements in 21 red giant branch stars in the metal-poor globular cluster NGC 6397 .
We compare the derived abundance patterns with theoretical predictions in the framework of the “ wind of fast rotating massive star ” -scenario .

Methods : High-resolution spectra were obtained with the FLAMES/UVES spectrograph on the VLT .
We determined non-LTE abundances of Na , and LTE abundances for the remaining 21 elements , including O ( from the [ OI ] line at 630 nm ) , Mg , Al , \alpha , iron-peak , and neutron-capture elements , many of which had not been previously analysed for this cluster .
We also considered the influence of possible He enrichment in the analysis of stellar spectra .

Results : We find that the Na abundances of evolved , as well as unevolved , stars in NGC 6397 show a distinct bimodality , which is indicative of two stellar populations : one primordial stellar generation of composition similar to field stars , and a second generation that is polluted with material processed during hydrogen-burning , i.e. , enriched in Na and Al and depleted in O and Mg .
The red giant branch exhibits a similar bimodal distribution in the Strömgren colour index c _ { y } = c _ { 1 } - ( b - y ) , implying that there are also large differences in the N abundance .
The two populations have the same composition for all analysed elements heavier than Al , within the measurement uncertainty of the analysis , with the possible exception of [ Y/Fe ] .
Using two stars with almost identical stellar parameters , one from each generation , we estimate the difference in He content , \Delta Y = 0.01 \pm 0.06 , given the assumption that the mass fraction of iron is the same for the stars .

Conclusions : NGC 6397 hosts two stellar populations that have different chemical compositions of N , O , Na , Mg , and probably Al .
The cluster is dominated ( 75 % ) by the second generation .
We show that massive stars of the first generation can be held responsible for the abundance patterns observed in the second generation long-lived stars of NGC 6397 .
We estimate that the initial mass of this globular cluster is at least ten times higher than its present-day value .