Abstract
It is evident from infrared observations made over the last two decades that there is a group of Wolf-Rayet stars which make dust episodically in their winds. In cases where observations have continued for long enough, it is seen that dust-formation episodes occur repeatedly, at intervals of about a decade. Three such episodes by the prototype, WR 140, have been observed and linked to the interaction of the winds of the WC7 and O4-5 components of this binary. A second episode by another system containing a WC7 star, WR 137, has recently begun. New optical spectroscopy of two other WC systems, not previously known to be binaries and which have showed dust-formation episodes, indicates the presence of OB companions and suggests that they too might be binaries. Closely related to these systems is the long-period WC9+B0I binary WR 70, whose dust emission is more complex. These systems can give new insights to the structure of Wolf-Rayet winds.
