Left panel: effect of increasing D _ext on the evolutionary tracks at two different initial stellar masses, using a D _mix profile as in Fig. 1b. The darker the colour of the track, the higher the radiative envelope mixing. Centre and right panels: two-dimensional surface plots illustrating the correlations between M _ini , D _ext, and dP (given in seconds) at two different X _c. The darker the colour, the higher the average period spacing value of the dipole prograde modes. The contour lines show positions of constant dP.