Using HII region spectra to probe the ionizing radiation from massive   stars
  We present some results of an on-going project aimed at studying a sample of Galactic HII regions ionized by a single massive star to test the predictions of modern generation stellar atmosphere codes in the H Lyman continuum. The observations collected for this study comprise the optical spectra of the corresponding ionizing stars, along with imaging and long-slit spatially resolved nebular observations. The analysis of the stellar spectra allows to obtain the stellar parameters of the ionizing star, while the nebular observations provide constraints on the nebular abundances and gas distribution. All this information is then used to construct tailored photoionization models of the HII regions. The reliability of the stellar ionizing fluxes is hence tested by comparing the photoionization model results with the observations in terms of the spatial variation across the nebula of an appropriate set of nebular line ratios. 
S. Sim\'on-D\'iaz (1,3), J. Garc\'ia-Rojas (2), G. Stasi\'nska (3) and   C. Esteban (4) ((1) Observatoire de Gen\`eve, (2) U...