Variations in the reduced χ² as a function of the radius r_c of the assumed discontinuity. The black curves correspond to the inversions performed with the best-fit models of Cesam2k and the gray curves, to the inversions performed with the best-fit models of astec. The circles indicate the lowest value of the χ² function. The vertical dot-dashed blue line indicates the location of the H-burning shell for the best-fit models.