Surface nitrogen abundance enhancement during MS evolution as a function of D _ext. Panel a: for an increasing radiative envelope mixing towards the surface of the stellar model based on the D _mix profile in Fig. 1d. The red dashed line shows the change in N¹⁴ for a constant envelope mixing (Fig. 1b). Panel b: corresponding effects of varying D _ext on the (l, m) = (1,1) period spacing series at X _c = 0.7, 0.5, and 0.1 (from top to bottom). See text for further explanation.