Empirical diagnostics of HII regions based on sulphur emission lines
  Spectrophotometry constitutes a unique way to obtain the whole diagnostics (density, temperature, ionic abundances) of ionized gas nebulae, thus providing invaluable information about the objects where they reside. If such nebulae are in the low excitation regime, the diagnostics have to relie on the observation of the more intense emission lines. We have compiled data for a sample of HII regions, GEHR and HII galaxies and compared them with results derived from photoionizacion models whose parameters cover the physical conditions of the nebulae. Our results confirm a lower uncertainty for the diagnostics when using empirical methods based on the strong lines of [SII] and [SIII] replacing and complementing those based on the [OII] and [OIII] lines. 
E. Perez-Montero and A.I. Diaz (Universidad Autonoma de Madrid, Spain)