**Table 4:** $R{_{\rm V}}$ values derived from optical and infrared photometry based on three different methods. The one but last column lists previously reported values for $R{_{\rm V}}$ . In Col. 2 we list the comparison stars (or spectral type) adopted to compute $R{_{\rm V}}$ for the targets in Col. 1 as discussed in the main text. Together with the reddening (Col. 3) three methods were used to derive $R{_{\rm V}}$ (Cols. 5-7). The average $R{_{\rm V}}$ is given in Col. 8, and a literature value is given in Col. 9 when available. The last Col. 10 gives the spectral type of the adopted comparison target or the foreground reddening based on the Galactic extinction curve ( $R_{\rm V} = 3.1$ ) that is subtracted from the constructed "total'' extinction curve.
$\begin{displaymath}{\begin{tabular}{lllllllrlll}\hline\hline & & & \multicolumn... ...25 & 4.29 & 4.26 (0.04) & & & B0~Ia \\ \hline \end{tabular} }\end{displaymath}$

References: Intrinsic infrared colours from: ^[1] Wegner (1994); ^[2] Johnson et al. (1966). ^[3] Gordon et al. (2003); ^[4] Morgan & Nandy (1982); ^[5] 2MASS (Cutri et al. 2003); ^[6] Clayton et al. (1983); ^[7] Savage & Fitzpatrick (1984).
^[a] Colour excess derived by adopting intrinsic colours for supergiants from Wegner (1994), Schmidt-Kaler (1982), Johnson et al. (1966); ^[b] Errors in average $R{_{\rm V}}$ column are based on sigma from average of the methods G03 (Gordon et al. 2003), F99 (Fitzpatrick 1999) and CCM89 (Cardelli et al. 1989), plus the typical error on $R{_{\rm V}}$ resulting from a 0.5 spectral type mismatch (assuming same luminosity class).