TITLE: GCN CIRCULAR NUMBER: 23656 SUBJECT: GRB 190109B: COATLI Optical Observations DATE: 19/01/09 16:11:22 GMT FROM: Alan M. Watson at Instituto de Astronomia UNAM Eleonora Troja (GSFC), Alan M. Watson (UNAM), Nat Butler (ASU), Rosa L. Becerra (UNAM), William H. Lee (UNAM), Margarita Pereyra (UNAM), Carlos Román-Zúñiga (UNAM), and Alexander Kutyrev (GSFC) report: We observed the field of GRB 190109B (Sbarufatti et al., GCN Circ. 23642) with the COATLI 50-cm telescope and interim imager at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir (http://coatli.astroscu.unam.mx) from 2019-01-09 11:57:47.6 to 13:27:45.2 (from 98.5 seconds to 1.53 hours after the trigger) at air masses above 3, obtaining a total of 59.1 minutes of exposure in the w filter. We do not detect an optical source within the enhanced XRT position region (Beardmore et al., GCN Circ. 23652) to a 5-sigma limiting magnitude of w = 20.3. Our w magnitudes are calibrated against the USNO-B1 catalog (adjusted to an approximate AB system) and are not corrected for Galactic extinction in the direction of the GRB. We thank the COATLI technical team and the staff of the Observatorio Astronómico Nacional.