Multiple ionisation sources in HII regions and their effect on derived
  nebular abundances
  We present a theoretical investigation of the effect of multiple ionisation
sources in HII regions on the total elemental abundances derived from the
analysis of collisionally excited emission lines. We focus on empirical methods
based on direct temperature measurements that are commonly employed in cases
when the temperature of the nebular gas can be determined from the ratio of
nebular to auroral lines of (e.g.) doubly ionised oxygen. We find that direct
temperature methods that employ a two-temperature zone approach (DT2T methods)
are very robust against the spatial distribution of sources. Errors smaller
than 0.15 dex are estimated for regions where the metallicity is twice solar
and errors below 0.05 dex for solar metallicities and below. The biases
introduced by the spatial distribution of the ionisation sources are thus much
smaller for DT2T methods than for strong line methods, previously investigated
by Ercolano, Bastian & Stasinska. Our findings are in agreement with the recent
study of HII regions in NGC 300 by Bresolin et al.
