Luminosity L, effective temperature T_eff and radius R of a representative selection of mass zones plotted against time t for the low-metallicity model series x54. The red line in the R panel shows the position of the grey-approximation optical depth τ_g = 2/3. T_eff in the grey pulsation model is defined as the temperature at τ_g = 2/3 and is close to the effective temperature ∝ (L/R²)^1/4 of the non-grey atmospheric stratification. The blue-shaded regions show the time intervals in which models were selected for computing detailed atmospheres (shown as red circles in the L panel).