Values of the four parameters, α,  β,  γ, and ϵ, for the different Wesenheit magnitudes considered in this work. Red dots show the results obtained by adopting the Fitzpatrick (1999) reddening law (F99 in the label). The shaded area comprises the range −0.4 < γ < −0.2 mag/dex, where most of the recent literature results are located. The units of α,  β,  γ,  ϵ and of their uncertainties are mag, mag/dex, mag/dex, and mas, respectively.