Curvature (α) as a function of the large frequency separation (Δν) for observations (left) and models (right), split for RGB stars (panels a–d) and CHeB stars (panels e–h), colour-coded by mass (panels a–b and e–f) and metallicity (panels c–d and g–h). The colour code is the same for both the observations and the models. The black lines indicate the RGB and CHeB fits from Fig. 2 and Eq. (7), with the coefficients from Table 1, except for panels c and d, where we use a fit through the selected observed values to account for the dependence of α on mass.