TITLE: GCN CIRCULAR NUMBER: 11255 SUBJECT: GRB 100901A: CrAO optical observations DATE: 10/09/07 18:32:18 GMT FROM: Alexei Pozanenko at IKI, Moscow V. Rumyantsev (CrAO), D. Shakhovkoy, A. Pozanenko (IKI) report on behalf of larger GRB follow-up collaboration: We observed the afterglow (Immler et al., GCN 11159) of the Swift GRB 100901A (Immler et al., GCN 11159) with AZT-11 telescope of CrAO observatory on September 3 (UT) 20:24 - 21:18 and 5 (UT) 22:39 - 23:51. The afterglow photometry is based on the same reference stars of SDSS DR7 used in our previous reports (GCNs 11191, 11200): T0+ Filter, Exposure, mag. (mid, d) (s) 2.3045 R 19x180 20.07 +/- 0.08 4.4045 R 18x180 21.10 +/- 0.20 The photometry is compatible with photometry at the same period (4.39 - 4.55 days) reported by Kann et al (GCN 11236).