Optical light curve in the $r$ band, as a function of time starting at UT May
8.904, from the Palomar data listed in Table 1 (solid squares).  The data have
been corrected for the Galactic extinction and converted to specific fluxes as
described in the text.  These numbers supersede our preliminary measurements
\refto{djo97b}. We also include a measurement \refto{mig97} of $R_C = 19.78 \pm
0.05$ mag (in the Kron-Cousins system) at the mean epoch of May 10.85, close to
the peak of the light curve (open circle).  After correcting for the extinction
and using the $R_C$ zero-point calibration \refto{bes90}, we derive $F_\nu =
45.9~\mu$Jy at the effective wavelength $\lambda_{eff} \approx 641$ nm, close
to the center of the $r$ band. We do not use other magnitudes reported in the
IAU Circulars so far, due to the lack of information about the photometric
systems used, photometric conditions, etc.  The decline in brightness seen
after May 10 is consistent with a power--law $F_\nu \sim t^{-1}$, with time
counting from the GRB event, shown as the dotted line drawn through our May 11
data point.