TITLE: GCN CIRCULAR NUMBER: 31003 SUBJECT: GRB 211023B: Mondy optical afterglow confirmation DATE: 21/10/26 11:09:14 GMT FROM: Alexei Pozanenko at IKI, Moscow <apozanen@iki.rssi.ru> A. Pozanenko (IKI), S. Belkin (IKI), E. Klunko (ISTP), N. Pankov (HSE), report on behalf of GRB IKI FuN: We observed the field of GRB 211023B (Dichiara et al., GCN 30960) with AZT-33IK telescope of Sayan observatory (Mondy) starting on 2021-10-23 (UT) 21:23:21. We obtained 60 images in R-filter. We confirm afterglow of the GRB 211023B (Dichiara et al., GCN 30960; Kuin et al. GCN 30975; Kann et al. GCN 30995; O'Connor et al., GCN 31000; Rastinejad et al., GCN 31002). Preliminary photometry of the afterglow in stacked images is following Date UT start t-T0 Filter Exp. OT Err. UL(3sigma) (mid, days) (s) 2021-10-23 21:23:21 0.15613 R 15*60 20.46 0.31 20.5 2021-10-23 21:23:21 0.03297 R 60*60 20.97 0.40 20.9 The photometry is based on the nearby USNO-B1.0 stars RA DEC R2 11:21:12.41280 +39:08:38.1120 15.33 11:21:05.20872 +39:04:33.9600 14.18 We note the obtained magnitudes are in agreement with non-detections of the optical counterpart (Lipunov et al., GCN 30964; Pozanenko et al., GCN 30972; Watson et al., GCN 30973; Yu et al., GCN 30976).