We present the first near-infrared images of dusty Wolf-Rayet star WR 98a .
Aperture masking interferometry has been utilized to recover images at the diffraction-limit of the Keck-I telescope , \stackrel { < } { { } _ { \sim } } 50 mas at 2.2µm .
Multi-epoch observations spanning about one year have resolved the dust shell into a “ pinwheel ” nebula , the second example of a new class of dust shell first discovered around WR 104 ( Tuthill , Monnier , & Danchi 1999a ) .
Interpreting the collimated dust outflow in terms of an interacting winds model , the binary orbital parameters and apparent wind speed are derived : a period of 565 \pm 50 days , a viewing angle of 35 \hbox { $ { } ^ { \circ } $ } \pm 6 ° from the pole , and a wind speed of 99 \pm 23 { mas } { yr } ^ { -1 } .
This period is consistent with a possible \sim 588 day periodicity in the infrared light curve ( ( ( Williams et al . 1995 ) ) ) , linking the photometric variation to the binary orbit .
Important implications for binary stellar evolution are discussed by identifying WR 104 and WR 98a as members of a class of massive , short-period binaries whose orbits were circularized during a previous red supergiant phase .
The current component separation in each system is similar to the diameter of a red supergiant , indicating that the supergiant phase was likely terminated by Roche-lobe overflow , leading to the present Wolf-Rayet stage .