TITLE: GCN CIRCULAR NUMBER: 21174 SUBJECT: GRB 170531B: CrAO confirmation of the afterglow DATE: 17/06/01 00:11:21 GMT FROM: Alexei Pozanenko at IKI, Moscow A. Pozanenko (IKI), D. Chestnov (ISON), A. Novichonok (ISON), A. Mokhnatkin (ISON) report on behalf of larger GRB follow-up collaboration: We observed the field of the GRB 170531B (Evans et al.,GCN 21171) with with Zeiss-1000/Koshka telescope starting on May 31 (UT) 22:32:32, i.e. ~30 minutes after burst onset. We took several images in filter R with exposures of 180 s. Near the border of XRT error circle (Evans et al.,GCN 21171) we detect the source in coordinates (J2000) 19:07:31.67 -16:25:09.3 with uncertainity of 0.5 arcsec in both coordinates. The source is probably coincedent with the object reported by (Lipunov et al., GCN 21172). The source is fading from R=19.1 at (UT) 22:32:32 down to R=19.5 at (UT) 23:30:56. Most probably the source is an afterglow of GRB 170531B.