Average ΔT_max derived from high time resolution light curves of V603 Aql, TT Ari and RS Oph as a function of isophotal wavelength of the comparison band. The broken lines are linear least-squares fits to the data points (in the case of RS Oph without considering the points referring to the W and U bands), weighted by the inverse of their standard deviations. The location of the reference band (B) is indicated by a cross. The error bars represent the mean error of the mean.