We present the discovery observations of the optical counterpart of the \gamma -ray burster GRB990712 taken 4.16 hours after the outburst and discuss its light curve observed in the V , R and I bands during the first \sim 35 days after the outburst .
The observed light curves were fitted with a power-law decay for the optical transient ( OT ) , plus an additional component which was treated in two different ways .
First , the additional component was assumed to be an underlying galaxy of constant brightness .
The resulting slope of the decay is 0.97 \pm ^ { 0.05 } _ { 0.02 } and the magnitudes of the underlying galaxy are : { V } = 22.3 \pm 0.05 , { R } = 21.75 \pm 0.05 and { I } = 21.35 \pm 0.05 .
Second , the additional component was assumed to be a galaxy plus an underlying supernova with a time-variable brightness identical to that of GRB980425 , appropriately scaled to the redshift of GRB990712 .
The resulting slope of the decay is similar , but the goodness-of-fit is worse which would imply that either this GRB is not associated with an underlying supernova or the underlying supernova is much fainter than the supernova associated with GRB980425 .
The galaxy in this case is fainter : { V } = 22.7 \pm 0.05 , { R } = 22.25 \pm 0.05 and { I } = 22.15 \pm 0.05 ; and the OT plus the underlying supernova at a given time is brighter .
Measurements of the brightnesses of the OT and the galaxy by late-time HST observation and ground-based observations can thus assess the presence of an underlying supernova .