Optical properties of the circumstellar dust around stars with aperiodic fadings

Multicolor observations of 21 rapid, irregular variable stars with Algol-like fadings and of R Coronae Borealis are used to derive the optical extinction coefficients of the circumstellar dust associated with these objects, θ(λ). We used more than 3600 uniform multi-color brightness measurements obtained in a rigorous U BV R system at the Terskol High-altitude Observing Station. The mean extinction coefficient θ for the circumstellar dust for this sample of stars coincides almost exactly with the interstellar extinction coefficient, θ_is, but is somewhat higher in the ultraviolet. We suggest an explanation for this difference.