Variation of the frequencies of five axisymmetric island modes (solid lines) as a function of the hydrogen mass fraction in the core with respect to that of the envelope. Here, X_env = X_ini = 0.739, as in Bouchaud et al. (2020). The horizontal dashed lines show the observed frequencies, while the vertical dashed line points out X_c = 0.972, which is the value where the frequencies of three 0⁺ modes of the model match the observed ones.