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 ∼ 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 ≿ 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.