F_red = F(833.0 nm) /F(634.8 nm) ratio calculated for the Class III spectra analyzed here (circles) and by MTR13 (crosses). Different colors refer to values of F_red calculated on spectra reddened with increasing values of A_V, from 0 mag to 5 mag, as reported in the legend. The data reported as A_V = −0.5 mag are obtained by de-reddening the spectrum by A_V = 0.5 mag. The black solid line is the value of F_red derived by HH14 for A_V = 0 mag, and the dotted line for A_V = 1 mag. The colored lines are our new estimates of this ratio, which are reported in Table B.1.