TITLE: GCN CIRCULAR NUMBER: 11270 SUBJECT: GRB 100901A: optical observations in Mondy and R-light curve DATE: 10/09/10 09:59:02 GMT FROM: Alexei Pozanenko at IKI, Moscow A. Volnova (SAI MASU), A. Pozanenko (IKI), E. Klunko, I. Korobtsev (ISTP) on behalf of larger GRB follow up collaboration report: We observed the Swift GRB 100901A (Immler et al., GCN 11159) with AZT-33IK telescope of Sayan observatory (Mondy) starting Sep. 01 (UT) 13:44:30 and continued observations within 2 nights after burst onset. Preliminary light curve of Mondy observation in R as well as R, r and nonfiltered observations inreported on behalf of our follow up collaboration (Klunko et al, GCN 11162; Andreev et al, GCN 11166; Andreev et al, GCN 11168; Elenin et al, GCN 11184; Andreev et al, GCN 11191; Andreev et al, GCN 11200; Andreev et al, GCN 11201; Elenin et al, GCN 11234; Rumyantsev et al, GCN 11255; Volnova et al, GCN 11266) can be found at http://grb.rssi.ru/GRB100901A/GRB100901A_lc.png The photometry calibration was performed against the same SDSS DR7 stars. There are some distinct features of the light curve. The tail of the flare reported earlier (Ivanov et al, GCN 11178), the rapid increase brightness after power law decay until 0.1d since trigger, the deep gap at ~0.13d, and the bump at ~0.3d corresponding to the bump in XRT light curve (Page et al, GCN 11171). We also confirm the late break between 5 and 6 days of the afterglow (Kann et al, GCN 11246; Volnova et al, GCN 11266). [GCN OPS NOTE(10sep10): Per author's request, the Subject line was changed from 100906A to 100901A.]