Ultraviolet observations of interstellar extinction near the Cepheus OB3 molecular cloud.

IUE data are used to derive UV extinction curves for O and B stars in the Cep OB3 association. These data are analyzed in conjunction with ANS photometry of additional association stars. While the level of the far-UV extinction differs greatly throughout the region the shape of the 2175 Å bump does not. The authors demonstrate that the different curves form a family and that the strength of the bump per E(B-V), E(BUMP)/E(B-V), can be used to parameterize the family. The implications of this result for the physics of interstellar grains are discussed. It is shown that the "bump index" can be used to reliably deredden the UV energy distributions of early-type stars if their intrinsic (B-V)₀ indices are known. It is emphasized that this analysis gives only a local solution to the reddening problem and that different regions must be analyzed on an individual basis.