Model of spiral density shells induced in a circumstellar outflow by orbital motion in a binary system. The gray scale shows the projected dust density for an observer lying in the system’s orbital plane—projected as a horizontal line running through the center. (The projection from most other angles has a spiral form.) To compare with the observations of IRAS 18276−1431, the dust distribution should be convolved with the illumination pattern, which is strongest along a cone centered on the system’s angular momentum axis—projected as a vertical line through the center. The details of this model are described separately (M. Morris et al. 2007, in preparation).