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 characteristics of UV extinction curves which differ from the standard extinction 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 Cardelli 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 small R-V are just as numerous as those with large Rv. The anomalous objects are arranged into two groups according to the strength of the bump at 0.217 mu. For a given value of ct this increases along the sequence: type A anomalous, normals and type B anomalous, suggesting that this sequence should correspond to an increase of the amount of small grains along the sightline. Considerations concerning the environmental characteristics indicate that the anomalous behaviour is not necessarily tied to the existence of dense gas clouds along the line of sight.