latter is shown in Figure 1c, although we consider this star to belong to Group IIa. Group IIb stars are shown in Figure 2c, from the 1997 data set.

The H$\alpha$ luminosity of Group I shells, presented in Figure 1, ranges between 1 and 43 $\times$ 10$^{\rm 36}$ ergs s$^{\rm
-1}$. In Figure 3 we show the distribution of H$\alpha$ luminosities as a function of the size of the nebulae, and plot the same distribution for the nebular structures surrounding WR stars in M33 (data from Drissen, Shara, & Moffat 1991). Further we indicate the detection limit in our data, with regard to shell-like objects. The minimum detectable luminosity for shells of varied size was determined empirically through the use of a simple code for modeling the anticipated appearance of idealized bubbles in a noisy background. Our program initially computes the projected surface brightness distribution of a shell (at very high spatial resolution) then transforms to an observable form by convolving with an appropriate Gaussian PSF and regridding. Finally, we scale the simulated image to units of emission measure. This result is added to a realistic noise background matching the properties of our observations. Inspection of the shell image with 