TITLE: GCN CIRCULAR NUMBER: 45669 SUBJECT: EP260920b: COLIBRÍ optical observations DATE: 26/09/21 04:32:10 GMT FROM: Leonardo Enrique García García Leonardo García-García (UNAM), Marion Guelfand (CPPM), Rosa L. Becerra (UNAM), Stéphane Basa (UAR Pytheas), Edilberto Aguilar-Ruiz (UNAM), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), Alan M. Watson (UNAM) report: We imaged the field of the EP260920b (Ni et al., GCN 45664) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-09-21 02:56 to 03:34 (from 12.81 to 13.47 hours after the trigger) and obtained 31 and 29 minutes of exposure in the r and z filters, respectively, under very poor conditions of transparency. The data were reduced and coadded with the ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction. In the stacked images, we marginally detect the optical counterpart reported by Liu et al. (GCN Circ. 45668), at a preliminary magnitude of: r = 20.9 +/- 0.3 Comparing our observations with those reported by Liu et al. (GCN Circ. 45668), the transient has faded, and we estimate a temporal decay index alpha ≈ 0.6. We note the presence of a bright spiral galaxy 2MASX J21422474-2802228 about 52 arcsec to the east of the counterpart. This galaxy is seen in the LS DR11 catalog (Dey et al. 2019) with a photometry of g = 16.822 +/- 0.001, r = 16.005 +/- 0.001, i = 15.630 +/- 0.001 and z = 15.344 +/- 0.001. We estimate a Pcc ≈ 0.01, making it a plausible candidate host galaxy. This galaxy has a spectroscopic redshift of z = 0.0519 from the 2dF Galaxy Redshift Survey (2dFGRS; Colless et al. 2001), corresponding to a projected offset of approximately 55 kpc if associated with EP260920b. We encourage spectroscopic follow-up of the optical counterpart to determine its redshift and test this possible association. We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM. COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.