Remarkable spectral variability in WR 104 (WC9): dust condensation in a hostile environment?

We present new observations of the WC9 star WR 104 (Ve 2-45), collected in 1996 July at the 2.5-m Isaac Newton Telescope, which reveal unprecedented spectral and photometric variations. Relative to previous observations by Torres & Massey, we find a simultaneous visual fading by similar to 1.1 mag, plus the disappearance of high-ionization spectral features (He II, C IV) with low-ionization features (He I, C II) relatively unchanged. We interpret this behaviour as obscuration of the inner Wolf-Rayet wind by a dust cloud condensation, analogous to R CrB stars, and recently proposed to explain occasional eclipses in other WC9 stars. Non-LTE model calculations for the WC9 component indicate a cloud diameter greater than or similar to 60 R. (20R*) - far smaller than in R CrB stars - probably formed at a radius beyond 300 R, (100R*). We detect the definite presence of a companion OB star which may facilitate the necessary conditions for dust formation via shocks.