TITLE: GCN CIRCULAR NUMBER: 21206 SUBJECT: GRB 170519A: Mondy optical observations DATE: 17/06/05 19:07:17 GMT FROM: Alexei Pozanenko at IKI, Moscow E. Mazaeva (IKI), A. Pozanenko (IKI), E. Klunko (ISTP), A. Volnova (IKI) report on behalf of larger GRB follow-up collaboration: We observed the field of GRB 170519A (Ukwatta et al., GCN 21106) with AZT-33IK telescope of Sayan observatory (Mondy) starting on May 20 (UT) 16:27:01. We took several images in R-filter with exposures of 120 s. The optical afterglow (Ukwatta et al., GCN 21106; Izzo, et al., GCN 21108) is clearly visible in single images. We continued our observations on May 27 (UT) 16:28:02, i.e., ~8.5 days after the BAT trigger. We took several R-filtered images. We do not detect the optical counterpart in the enhanced XRT error circle (Beardmore et al., GCN 21107). Preliminary photometry of the afterglow is following: Date UT start t-T0 Filter Exp. OT Err. UL (mid, days) (s) (3 sigma) 2017-05-20 16:27:01 1.47708 R 10*120 19.99 0.03 22.8 2017-05-20 17:07:02 1.50487 R 10*120 19.99 0.03 22.6 2017-05-27 16:28:02 8.49905 R 39*120 n/d n/d 22.6 The photometry is based on nearby SDSS stars reported by Mazaeva et al. (GCN 21169).