thresholds above the sky level in each of $R$ and $I$. Object X was higher than the threshold only in $I$, although it is visible by eye in $R$. We estimated its $R$ magnitude by fixing its position to that found in $I$, and comparing its brightness with that of C, whose $R$ and $I$ magnitudes were well determined.

We converted the instrumental magnitudes to Johnson $R$ and $I$ with the method of Holtzman  (1996). We used their calibrated zero-points as appropriate for PC1 and for each filter, and the corrections necessary for our nominal CCD gain. We set the magnitude scale for each band using color corrections derived from the ground-based colors of the images of the core and of G. For the images of the core, the mean is $R-I \approx 1.8$; for G, $R-I \approx 1.6$, both based on AVHM94. Our magnitude estimates follow from the instrumental magnitudes within circular apertures 5 pixels in radius ($\sim 0\farcs23$), except for that of G, which is clearly not point-like (see below), and required a radius of 20 pixels. Tables 1 and 2 show the coordinates and magnitudes of the images of the core, the lens galaxy G, object X, 