Résumé
S-type stars are late-type giants whose atmospheres are enriched in carbon and s-process elements because of either extrinsic pollution by a binary companion or intrinsic nucleosynthesis and dredge-up on the thermally-pulsing AGB. A large grid of S-star model atmospheres has been computed covering the range 2700 <= T-eff(K) <= 4000 with 0.5 <= C/O <= 0.99. ZrO and TiO band strength indices as well as VJHKL photometry are needed to disentangle T-eff, C/O and [s/Fe]. A "best-model finding tool" has been developed using a set of well-chosen indices and checked against photometry as well as low- and high-resolution spectroscopy. It is found that applying M-star model atmospheres (i.e., with a solar C/O ratio) to S stars can lead to errors in T-eff up to 400 K. We constrain the parameter space occupied by the S stars of the vast Henize sample in terms of T-eff, [C/O] and [s/Fe].