Hertzsprung–Russell (HR) diagram for SMC stars (i.e. Z = 0.002; top panel) and LMC stars (i.e. Z = 0.006; bottom panel) with evolutionary tracks of different masses between the ZAMS and the TAMS. The sub-surface convection zone stability windows as defined by the region for which Ra < 0.001 Ra_crit (cf. Eq. (1)) are shown in yellow for models with minimum mixing and blue for the additional models with maximum values of mixing. Evolutionary tracks with minimum and maximum mixing are shown as solid and dashed lines, respectively. The location of the sub-sample of stars with reliable TESS light curves, all of which have significant SLF variability, are shown as red points.