Ratio over for an isothermal (γ = 1) gas with different electron temperatures, indicated by c_s, and the weak-shock approximation (Mach number = 1), according to Eq. (4) in M04 (dashed lines). Additionally given are the correction factors over as they follow from radiation-hydrodynamics sequences for nebulae around different central-star models (cf. Perinotto et al. 2004; SJS): 0.595 M_⊙ (red dots), 0.605 M_⊙ (diamonds), and 0.625 M_⊙ (blue stars). Here R_rim is the rim-shock radius, Ṙ_rim its expansion speed, and the line of sight Doppler rim expansion speed from the emission-line peak separation of [O iii], all taken or computed from the model nebulae and shown until the maximum stellar temperature. The downward “spikes” seen in the 0.605 M_⊙ and 0.595 M_⊙ sequence (e.g. at V_rim ≃ 13–14 km s^-1 and 21–22 km s^-1) are due to the sudden ionisation front (He i/He ii) passage through the rim/shell interface. Afterwards, the electron temperature, hence also R, is a bit higher.