Pinwheel nebula around WR 98a

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, ~<50 mas at 2.2 micron. 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 +/- 50 days, a viewing angle of 35 +/- 6 degrees from the pole, and a wind speed of 99 +/- 23 mas/yr. This period is consistent with a possible ~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.