Schematic representation of the method used to compare X-ray and optical data for each temporal bin. The minimum and maximum optical fluxes allowed by the standard afterglow model assuming that there is a single break ν_opt < ν_b < ν_X are indicated by F_m,opt and F_M,opt, respectively. We derived F_X, F_opt, and the X-ray spectral slope from spectral analysis. Therefore, if F_m,opt < F_opt < F_M,opt, we then considered that the optical counterpart is consistent with a single synchrotron spectrum, and we could derive ν_b. The blue line has no break, the orange line has ν_b = ν_X, and the green line has a break between the optical and X-ray bands. We note that our assumption here is that ν_m < ν_opt, and it is supported by the observation of β_opt > 0 for all the analysed events (see Sect. 3.2) that exclude the case ν_b = ν_m.