of these two measurements. We defined irregularly shaped regions for shell fragments, such that the boundary traced the H$\alpha$ emission associated with a fragment. Masks containing shell fragments therefore tend to be arc shaped. Pixels within the regions defined above were summed to produce the total shell+background fluxes. The background was estimated from one or more polygonal regions immediately surrounding an shell (not shown), and were generally 2-3 times larger in area than the regions used to compute the shell+background fluxes. The background was then subtracted to produce a total shell H$\alpha$ flux. Uncertainties in the flux were estimated by adding in quadrature the rms background uncertainty, for the number of source pixels, and root-N photon noise.

The sizes of shells were estimated by measuring several (5-10) radial distances from a WR star to the outer edge of its surrounding shell, and averaging these values. For a given shell, the uncertainties in the measurements varied depending on factors such as the completeness and/or ellipticity of the shell. Radii were rounded to the nearest 5 pc, and have typical uncertainties of $\pm$5pc.

Figure 1a contains H$\alpha$+\[\] and blue (B-band) images of Group Ia shells from the 1997 data set. The regions 