Left panel: fraction of time spent by SESs in systems similar to φ Persei (see Sect. 4.4) in the helium core burning phase (pale blue) and the helium shell burning phase (bright blue). Right panel: fraction of time spent by SESs in systems similar to φ Persei below (pale green) and above (bright green) the luminosity limit L_obs. Before adding up all the time steps in the grid to obtain the plots above, each time step is weighted by the likelihood of fit from the derived initial parameters.