TITLE: GCN CIRCULAR NUMBER: 11197 SUBJECT: GRB 100901A: HCT optical observations DATE: 10/09/03 19:52:42 GMT FROM: D.K. Sahu at Indian Inst of Astrophysics,Bangalore D.K. Sahu, Sonam Arora, N. Shantikumar Singh and Sreeja S. Kartha (IIA, Bangalore) report. We observed the optical afterglow of GRB 100901A (Immler et al., GCN 11159) in Bessell V, R, and I bands with the 2m. Himalayan Chandra Telescope of the Indian Astronomical Observatory, Hanle, India. Observations were made between 18:43 UT and 19:50 UT on 02/09/2010. The optical afterglow was detected in our individual frames of 300sec each in R and I bands. The preliminary R and I magnitudes of the optical afterglow calibrated using USNO B1.0 1127-0027231 reference star (R=15.37, I=15.13) is as under: Mid UT Exposure Filter Mag 18:50 3X300sec R 18.71 +/- 0.10 19:12 3X300sec I 17.51 +/- 0.15