TITLE: GCN CIRCULAR NUMBER: 21382 SUBJECT: GRB 170728A: LCO observations DATE: 17/07/29 14:47:33 GMT FROM: Antonino Cucchiara at UVI A. Cucchiara, D. Morris (U. of Virgin Islands), and C. Guidorzi (U. Ferrara), reports on behalf of a larger collaboration: "On July 28.38 UT (T_0 +2.1h) we began observing the center of the field of GRB 170728A (Cannizzo et al. GCN 21367, Evans et al. GCN 21368) using the Las Cumbres Observatory 1m telescope in Cerro Tololo. We performed a series of 4x240s observations in R and Ic band for a total of 16 minutes on sky in each filter at average airmass of 1.8. Further observations were requested in Sloane i' filter for a total of 360s. In the full combined R and I+i' stacks we identified no optical counterpart within the center of Swift-XRT refined position (Evans et al. GCN 21368) at the following 3-sigma limits: R > 18.9 mag I > 18.4 mag These magnitudes are calibrated against nearby USNO-B1 sources, and are not corrected for Galactic extinction."