Left panel: distribution of galaxies and clusters in the log(⟨I⟩_e)−​log(R_e) plane. Black filled circles indicate our BCGs, red filled circles our II-BCGs, green filled circles our random sample of normal ETGs, gray filled dots the 34982 galaxies analyzed by D’Onofrio et al. (2014) with GASPHOT, empty magenta stars the faint ETGs of Bettoni et al. (2016), and filled blue circles our clusters. The dashed line with slope −1 is that predicted for virialized systems for a possible ZoE. The zero point of this line was chosen as explained in Sect. 4.4. The dotted line is that expected for systems of equal luminosity M_V = −21.5 with slope −2. The solid line with slope −1.2 is that obtained by Capaccioli et al. (1992). Right panel: log(⟨I⟩_e)−​log(R_e) plane with real and simulated objects. The color-coding and symbols are the same as before. The open circles with the same colors of real galaxies are used for simulated objects. The plotted lines are the same as in left panel.