TITLE: GCN CIRCULAR NUMBER: 21389 SUBJECT: GRB 170728B: COATLI Observations And Confirmation of Fading DATE: 17/07/30 02:20:58 GMT FROM: Alan M. Watson at Instituto de Astronomia UNAM Alan M. Watson (UNAM), Nat Butler (ASU), Rosa L. Becerra (UNAM), Carlos Román-Zúñiga (UNAM), and William H. Lee (UNAM) report: We observed the field of the short GRB 170728B (Cenko et al., GCN Circ. 21371) with the COATLI 50-cm telescope and interim imager (Watson et al. 2016, Proc. SPIE, 9908, 50) at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir from 2017-07-29 04:21 to 08:19 obtaining a total of 2.76 hours of observation in the clear filter. We detect the source reported by D’Avanzo et al. (GCN Circ. 21372). Calibrating with respect to the USNO-B1 catalog, we estimate a preliminary magnitude of R = 22.40 +/- 0.16. This magnitude is in the USNO-B1 Vega system and is corrected for Galactic extinction in the direction of the GRB. The source has faded by around two magnitudes between the observations in the first hour or so (D’Avanzo et al., GCN Circ. 21372; Heintz et al, GCN Circ. 21374; and Guidorzi et al., GCN Circ. 21375) and our observations. This confirms that the source is the optical transient associated with the GRB. We thank the COATLI technical team (Fernando Ángeles, Oscar Chapa, Salvador Cuevas, Alejandro Farah, Jorge Fuentes, Rosalía Langarica, Fernando Quirós, and Carlos Tejada) and the staff of the Observatorio Astronómico Nacional.