Equivalent radius-to-25 mag arcsec^-2 isophote radius ratio as a function of the radial velocity of galaxies. a) Comparison between spectra taken from Martínez et al. (2010) and SDSS for the HCG sample and b) comparison between the HCG and CIG samples. In each panel, the lines join the mean values determined in sets of a) 20 and b) 30 galaxies at different redshifts. The error bars give the standard deviation of the means.