Classification and properties of UV extinction curves

The catalog of Savage et al. (1985) reporting colour excesses of 1415 stars from ANS photometry offers the opportunity to deeply investigate the chara cteristics of UV extinction curves which differ from the standard extinctio n of the diffuse interstellar medium. To this aim we have selected a sample of 252 curves, which have been compared with the relations derived by Card elli et al. (1989; CCM in the following) for a variety of Rv values in the range 2.4-5 and have been classified as normal if they fit at least one of the CCM curves or anomalous otherwise. We find that normal curves with smal l R-V are just as numerous as those with large Rv. The anomalous objects ar e arranged into two groups according to the strength of the bump at 0.217 m u. For a given value of ct this increases along the sequence: type A anomal ous, normals and type B anomalous, suggesting that this sequence should cor respond to an increase of the amount of small grains along the sightline. C onsiderations concerning the environmental characteristics indicate that th e anomalous behaviour is not necessarily tied to the existence of dense gas clouds along the line of sight.