arc between images A2 and B, but at a modest level of significance. If such invariance were confirmed with high significance, we might be able to usefully test detailed extinction models. Reddening in G would likely be patchy at the same level as the observed variations among the images of the core (RMS $\sim 0.13$ mag) over the length of the arc, and should affect the latter, as well as the former. Therefore, if our measurements were confirmed with greater accuracy, they would suggest that (at least) the bulk of the reddening occurs in the host galaxy of the QSO, rather than in G. The large discrepancies between the $R$ ($2.50 \pm 0.04$), $I$ ($2.11 \pm 0.01$) and radio ($1.13 \pm 0.03$) flux ratios A1/A2 would then imply that microlensing is important on at least one of the paths from the core to images A1 and A2, as suggested in WMS95. Optical/IR monitoring observations could reveal microlensing, but it is possible that A2, for instance, is and would remain in a “low” state for a period of years; disentangling the possibilities could then require a very long temporal baseline.

In each of our models B-D, the estimated ellipticity of the 