M33. Upper panel: individual and binned results for H ii regions (upper panel) and PNe (lower panel) in M33. The O abundances of PNe are from Magrini et al. (2009b) and Bresolin et al. (2010; blue stars) and of H ii regions from Magrini et al. (2007, 2010) and references therein, and Bresolin et al. (2010; red crosses). The continuous black lines are the gradient of Magrini et al. (2010) for H ii regions (their whole sample) and the non-Type I PN sample of Magrini et al. (2009b) in the upper and lower panels, respectively. Lower panel: weighted linear fits of PN and H ii region binned metallicities. Symbols and curves as in Fig. 6.