High resolution abundance analysis of 16 giants and subgiants in the metal-poor globular cluster NGC 6397

High-resolution echelle spectra of five giants and eleven subgiants of the metal-poor globular cluster NGC 6397 were obtained at the ESO 3.5m (NTT) an d 3.6m telescopes. A detailed analysis of the 16 sample stars was carried out, providing atmos pheric parameters (T-eff, log g, [Fe/H], v(t)) and element abundances. The metallicity of the cluster is [Fe/H] = -2.0 with a r.m.s. of +/-0.05, i rrespective of the position in the Colour - Magnitude Diagram, with no vari ations as a function of effective temperatures and gravities. The dispersio n in metal abundance, obtained over a range of 5 magnitudes and 1400 K, is negligible and well within the measurement uncertainties. alpha-elements are mildly overabundant while the s-elements are deficient r elative to iron by about 0.2 dex. [O/Fe] is also mildly overabundant for th e two stars where the line is detected. The Li abundances in the present sample and those of turnoff stars by Pasqu ini and Molaro (1996) are complementary in terms of evolutionary stage. The Li abundances decrease off the main sequence along the red giant branch, c onfirming that extra depletion must occur for stars cooler than similar to 5000 K and V brighter than similar to 14.