TITLE: GCN CIRCULAR NUMBER: 11254 SUBJECT: GRB 100906A: RTT150 optical observations DATE: 10/09/07 18:30:39 GMT FROM: Rodion Burenin at IKI, Moscow A. Tkachenko (IKI), I. Khamitov (TUG), R. Burenin, M. Pavlinsky, R. Sunyaev (IKI), I. Bikmaev, N. Sakhibullin (KSU/AST), Z. Eker (TUG), U. Kiziloglu (METU), E. Gogus (Sabanci Uni.) The optical counterpart of GRB 100906A (GCN 11227) was observed with Russian-Turkish 1.5-m telescope (RTT150, Bakirlitepe, TUBITAK National Observatory, Turkey), starting at Sep 06, 19:28 UT, i.e. ~5.64 hours after the burst, using TFOSC. Three series of BRI frames were made under moderate weather conditions. The afterglow is clearly detected in all images. Assuming the star at RA = 01:54:46.82, Dec. = +55:37:55.3 to have USNOB1.0 R2MAG = 15.70, we estimated the following magnitudes for the OT on combined images of every set: t-t0 band mag err 6.048 h R 18.80 0.02 9.610 h R 19.08 0.02 13.554 h R 19.69 0.04 The color of the afterglow changes from (B-R)=1.21 to (B-R)=1.31 at the beniginning and at the end of our observations.