only if the shell were to begin expanding from the surface of the star. Shells, however, are likely to form far from the surface of the star. In the intervening space between the star and the radius at which the shell is formed, the stellar wind (or possible ejecta) may expand at a much higher velocity than the observed expansion velocity of the resulting shell. Further, one must consider mass loss from pre-WR phases. The winds of massive stars can operate on timescales of several Myr (the typical lifetime of an O star), affecting the ISM up to radii of tens of parsecs. As for the gap at small shell radii, there may be unresolved circumstellar shells ($\leq$ 5-7 pc) surrounding WR stars in our data (see below). The point source detection limit is log L$_{\rm H\alpha}$ $\sim$ 34.5, well below the scale in Figure 3. We note that the upper limit of possible shell emission surrounding Group IIb stars also falls below this scale. The gap therefore remains somewhat surprising.

The implied Lyman continuum luminosity of Group I shells ranges from 0.8 to 31.8 $\times$ 10$^{\rm 48}$ photons s$^{\rm -1}$. Esteban et al. (1993) have estimated that Galactic WR 