R_V–λ_max plane. The red dots mark the CCM R_V values as a function of λ_max determinedby best-fitting the IR extinction curves with the CCM extinction law, the blue dots mark the R_V values determined by fitting the WD01 model to all observations, and the green triangles mark the R_V values from Wegner (2002) determined by the extrapolation method (see Table 2). The black line shows the R_V relationship from Whittet & van Breda (1978) and its 1σ uncertainty.