Mean nebular [O iii] electron temperatures vs. nebular line ratio L(4686)/L(Hβ) for the dynamical α = 3 sequences from Fig. 18 (dotted in thermal equilibrium). Again, the sequences are only plotted until maximum stellar temperature in order to avoid confusion. The circles give the corresponding positions of the Halo PNe: filled circles are based on the observed [O iii] lines, open circles are results of detailed analyses using photoionisation models (Howard et al. 1997). The inset renders an enlarged view of the region around L(4686)/L(Hβ) ≃ 1.