TITLE: GCN CIRCULAR NUMBER: 21183 SUBJECT: GRB 170531B: LCO observations DATE: 17/06/01 15:49:04 GMT FROM: Antonino Cucchiara at UVI A. Cucchiara, D. Morris (U. of Virgin Islands), C. Guidorzi (U. Ferrara), reports on behalf of a larger collaboration: "On June 1.00 UT (T_0 +2.0h) we began observing the center of the field of GRB 170531B (Evans et al. GCN 211171) using the Las Cumbres Observatory 1m Sutherland facility. We performed a series of 10x60s observations in R and I band for a total of 10 minutes on sky in each filter. Seeing conditions were poor. We identified no optical counterpart within the center of Swift-XRT refined position (Evans et al. GCN 21175), or at the location of the identified counterpart (Lipunov et al. GCN 21173, Pozanenko et al. GCN 21174, Schady et al. GCN 21178) at the following 3-sigma limits: R > 19.9 mag I > 19.6 mag These magnitude are calibrated against nearby USNO-B1 sources, and are not corrected for Galactic extinction."