Atomic diffusion coefficient of He just below the base of the convective envelope for Pop II and Pop I models (open and filled circles respectively) at the ZAMS and in the middle of the MS when X_C = 0.5X₀ (upper and bottom panels respectively). The [Fe/H] value of the models is colour-coded. With black five-pointed stars, we show the predicted values for the model with [Fe/H] = −5.8 computed for the most Fe-poor star J0023+0307 (Sect. 4.1.1 and 4.1.5).