Two projections of the stellar mass fraction HP: one with log(σ_e8) as a function of log(R_e)−γ − βlog(I_e) (left panel) and one with $log(f_{\mathrm{e}}^{\star })$ as a function of log(σ_e8) (right panel). The color of the points indicates values of log(f_e^⋆) and log(I_e). Both plots highlight the dependence on the stellar mass fraction. The one on the left, in particular, is another way to plot the FP and shows how ETGs with similar stellar mass fractions seem to lie on planes with different slopes and intercepts. The isolated object with a peculiarly low value of f_e^⋆ is the one with n = 7.86 described in Appendix C.