  A detailed spectroscopic study, from lambda 3700 A to 1 um, was performed for
a sample of 34 HII galaxies in order to derive fundamental parameters for their
HII regions and ionizing sources, as well as gaseous metal abundances. All the
spectra included the nebular [SIII]9069,9532 A lines, given their importance in
the derivation of the S/H abundance and relevant ionization diagnostics. A
systematic method was followed to correct the near-IR [SIII] line fluxes for
the effects of the atmospheric transmission.A comparative analysis of the
predictions of the empirical abundance indicators R23 and S23 has been
performed. The relative hardness of their ionizing sources was studied using
the eta parameter, and exploring the role played by metallicity. For 22
galaxies of the sample a direct value of te[SIII] was derived, along with their
ionic and total S/H abundances. Their ionic and total O/H abundances were
derived using direct determinations of te[OIII].For the rest of the objects,
the total S/H abundance was derived using S23. The abundance range covered by
our sample goes from 1/20 solar up to solar metallicity. The mean S/O ratio
derived is log (S/O)=-1.68+/-0.20 dex, 1 sigma below the solar S/O value. The
S/O abundance ratio shows no significant trend with O/H over the range of
abundance covered in this work, in agreement with previous findings.There is a
trend for HII galaxies with lower gaseous metallicity to present harder
ionizing spectra. We compared the distribution of the ionic ratios O+/O++ vs.
S+/S++ derived for our sample with the predictions of a grid of photoionization
models performed for different stellar effective temperatures. This analysis
indicates that a large fraction of galaxies in our sample seem to be ionized by
extremely hard spectra.
