WR 121 obscured by a dust cloud: the key to understanding occasional ``eclipses'' of ``dusty" Wolf-Rayet WC stars?

We observed the Wolf-Rayet star WR 121 (= AS 320, WC9) coming out of a minimum with a depth of 0fm8 to its normal brightness in a dozen days. The nature of this event was analyzed by means of Walraven five-colour photometry. The colour changes are similar to those caused by interstellar dust. Hot dust is known to be formed continuously around this carbon-rich Wolf-Rayet star. Therefore, we suggest that the fading of WR 121 was caused by the temporary condensation of an optically thick line-of-sight dust cloud, comparable to what occurs around R Coronae Borealis stars. We suggest that occasional ``eclipses'' shown by other ``dusty" Wolf Rayet stars (WR 113, WR 103) are also caused by such temporary condensing dust clouds. In addition, we present observations of a new ``eclipse'' of WR 103. This brightness dip was more shallow than the earlier ``eclipses'' and the star reddens during the descent. This also supports the model of a condensing dust cloud. >From modeling the shapes of the various ``eclipses'' we find that the condensation takes place at radii ranging from 80 to 800 R<sub>sun</sub>, i.e. between the stellar surface and the permanent dust shell that is inferred from the infrared excess. >From the colour changes we estimate the sizes of the particles in the clouds to be of order 0.1 mu m and using the depth of the darkening we derive a dust mass condensation rate per column in the range of 3 -- 20 10(-10) kg m(-2) s(-1) . The dust mass flux per solid angle turns out to be comparable to that of the shell. Moreover, we find two possible trends within our small set of ``eclipses'': (1) the closer the condensation occurs to the star, the larger the dust mass flux is, and (2) the closer the condensation, the larger the particles are. These correlations are discussed within the framework of the model. Based on observations collected at the European Southern Observatory (ESO), La Silla, Chile