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), collecte d in 1996 July at the 2.5-m Isaac Newton Telescope, which reveal unpre cedented spectral and photometric variations. Relative to previous obs ervations 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) rela tively unchanged. We interpret this behaviour as obscuration of the in ner Wolf-Rayet wind by a dust cloud condensation, analogous to R CrB s tars, and recently proposed to explain occasional eclipses in other WC 9 stars. Non-LTE model calculations for the WC9 component indicate a c loud diameter greater than or similar to 60 R. (20R) - far smaller th an 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 faci litate the necessary conditions for dust formation via shocks.