J. P. Halpern (Columbia), and R. Fesen (Dartmouth) report on behalf
of the MDM Observatory GRB follow-up team that the optical afterglow of
GRB 980613 discovered by Hjorth et al. (GCN #109) is also present and
variable on MDM images at a level just below the limit of I = 22.3
previously quoted by us in GCN #106.
 
Our J2000 position of the OT is (+/- 0.6"):
 
RA  =  10 17 57.82
Dec = +71 27 25.5
 
Measured magnitudes (at mean epoch) and 1-sigma statistical errors are:
 
R = 22.96 +/- 0.09  (June 14.24 UT)
I = 22.53 +/- 0.09  (June 14.20 UT)
I = 22.83 +/- 0.15  (June 15.19 UT)
 
Actual errors are larger, limited by systematic effects of fringing in the
I band.  Photometry was calibrated using Landolt standards.  Our magnitudes
of the five reference stars measured by Diercks et al. (GCN #108) are given
in the following table, and are in agreement with their values, as well as those
of Djorgovski et al. (GCN #117).  Quoted uncertainties are 1-sigma statistical.
Coordinates are measured with respect to the USNO A1.0 reference system.
 
Star    RA(2000)     Dec(2000)            R                 I
--------------------------------------------------------------------
 1    10 17 47.52   +71 26 59.9     17.91 +/- 0.01    17.34 +/- 0.01
 2    10 17 54.87   +71 27 39.8     19.20 +/- 0.01    18.76 +/- 0.01
 3    10 18 06.61   +71 27 04.7     20.02 +/- 0.05    18.72 +/- 0.01
 4    10 17 55.31   +71 28 16.3     19.54 +/- 0.02    18.61 +/- 0.01
 5    10 17 41.44   +71 28 08.9     18.70 +/- 0.01    18.31 +/- 0.01
--------------------------------------------------------------------
 
Our measurement of R = 22.96 for the OT on June 14.24 is similar to that
of Hjorth et al. (R = 22.9 +/- 0.2 on June 13.89), which may be consistent
with a plateau in the first 24 hours after the burst, or perhaps just an
effect of the large error bars.  Our R-band measurement combined with the
later detection by Djorgovski et al. (R = 24.5 +/- 0.5 on June 16.30)
implies a power-law decay slope of 1.3.
 
Our images are posted at http://cba.phys.columbia.edu/grb/980613/.