TITLE: GCN CIRCULAR NUMBER: 10818 SUBJECT: GRB 100526A: optical upper limit DATE: 10/06/01 20:17:14 GMT FROM: Alexei Pozanenko at IKI, Moscow D. Shakhovkoy, V. Rumyantsev (CrAO), A. Pozanenko (IKI) report on behalf of larger GRB follow-up collaboration: We observed the field of the Swift GRB 100526A (Vetere et al. GCN 10797) with AZT-11 telescope of CrAO between (UT) May 26 21:34 -- 22:36 under mean seeing of about 3 arcsec. No objects is detected in enhanced XRT error circle (Beardmore et al. GCN 10800). The upper limit in a stacked image based on USNO-B1.0 star 1156-0223950 (assuming R=18.44) is following: T0+ Filter, Exposure, mag. (mid, d) (s) 0.2354 R 20x180 > 21.4