Top panel: R_A as a function of the luminosity. Green circles depict the secure group candidates in Foëx et al. (2013), green diamonds those with 2.0′′ ≤ R_A ≤ 8.0′′. Bottom panel: R_A as a function of optical richness. The quantities were measured within an aperture of 0.5 Mpc. The green continuous line shows the fit to the green diamonds, with the 1σ-error depicted as a gray shaded region. Blue asterisks with error bars are plotted to highlight the correlation between R_A and luminosity or optical richness after binning the data. The dotted line shows the relations reported by Zitrin et al. (2012) with a dispersion depicted by the two dashed lines (see Sect. 5.2).