Left panel: mean density vs. core conditions indicator (t_u) plot. The inversion results are plotted in orange with their respective error bars. The crosses are values for the reference models computed with the Levenberg-Marquardt algorithm with AGSS09, the black × shows one example of a model computed with GN93 and Y_f = 0.25. Right panel: surface chemical composition box for 16CygB. The colour code used allows direct trend comparisons between the surface chemical composition and the t_u values. The size of the symbols is related to the intensity of microscopic diffusion, the smaller the symbol, the smaller the D coefficient. The + and the ◇ illustrate the impact of the metallicity on the t_u value.