TITLE: GCN CIRCULAR NUMBER: 21837 SUBJECT: GRB 170906C: COATLI Observations DATE: 17/09/06 19:47:21 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 GRB 170906C (Siegel et al., GCN Circ. 21826) with the COATLI 50-cm telescope and interim CCD imager (Watson et al. 2016, Proc. SPIE, 9908, 50) at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir from 2017-09-06 08:17 to 12:11 obtaining a total of 2.93 hours of observation in the clear w filter. We do not detect a source within the enhanced XRT error region reported by Evans et al. (GCN Circ. 21831). Calibrating with respect to the USNO-B1 catalog, we estimate a 3-sigma upper limit of: w > 21.7 This magnitude is in the AB system and is not corrected for Galactic extinction in the direction of 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.