  S-type stars are late-type giants whose atmosphere is 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 < Teff < 4000 K with 0.5 < C/O < 0.99. ZrO and TiO band strength indices
as well as VJHKL photometry are needed to disentangle Teff, C/O and [s/Fe]. A
"best-model finding tool" was 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 on Teff up to 400K. We constrain the parameter
space occupied by S stars of the vast sample of Henize stars in terms of Teff,
[C/O] and [s/Fe].
