Slope, α, and intercept, β (see Equation (2)), as a function of metallicity for the P−L relation for O24 models on the second and third crossings of the IS. In the bottom panel, linear fits determine the metallicity effect, γ. Open symbols (and fits plotted with dashed line) correspond to the P−L relation of fixed slope. In the bottom panel, relations for the second crossing were shifted by 0.1 for better visibility.