HRD of the γ Dor and SPB (left panel), and δ Sct and β Cep (right panel) stellar samples. The colour gradient indicates the stellar surface gravity, and over-plotted are MIST evolutionary tracks (Choi et al. 2016). Solid lines indicate instability strips for gravity modes in the left panel and pressure modes in the right panel. The red lines show the edges of the theoretical γ Dor instability strip from Dupret et al. (2005) (corresponding to a mixing length of respectively α_MLT = 2 (solid lines) and α_MLT = 1.5 (dashed lines) in the left-hand plot). The orange lines represent the observational δ Sct instability strip from Murphy et al. (2019). The blue and purple lines are the theoretical instability regions of SPB and β Cep stars, respectively, calculated by Burssens et al. (2020).