The chemical composition of H II regions in the outer Galaxy

New spectroscopic observations, including the optical and near-infrared ranges, have been obtained for a sample of HII regions located towards the Galactic anticentre. The sample includes H II regions with known galactocentric distances, their ionizing stars having been clearly identified from previous work. The spectra correspond to low-to-intermediate excitation nebulae, ionized by one/few late O/early B star(s). The global parameters of the regions, as well as their physical parameters such as electron density and temperature, are presented. Abundances have been computed using direct (spectroscopic) determinations of the electron temperature for some of the objects or alternatively assuming an adopted electron temperature derived from photoionization model fitting. Overall, the data appear to show a substantially flat gradient for the nitrogen abundance, going up to 18 kpc of galactocentric distance, as well as a mild gradient, if any, for oxygen. The data points for sulphur, although less numerous, give abundances which are consistent with the behaviour observed for the other elements. The N/O ratio also presents a very flat gradient across the outer Galaxy, with a value similar to the one derived from observations of optical emission lines of H II regions in the solar neighbourhood.