TITLE: GCN CIRCULAR NUMBER: 30781 SUBJECT: GRB 210905A, GROND observations DATE: 21/09/06 17:04:00 GMT FROM: Sylvio Klose at TLS Tautenburg <klose@tls-tautenburg.de> A. Nicuesa Guelbenzu, S. Klose (both TLS Tautenburg), and A. Rau (MPE Garching) report: We observed the field of GRB 210905A (Swift trigger #1071993; Sonbas et al., GCN 30765) simultaneously in g'r'i'z'JHK with GROND mounted at the 2.2m MPG telescope at La Silla Observatory (Chile). Observations started at 23:35 UT on September 5, about 23.2 hr after the GRB trigger. They were performed at an average seeing of 1.7 arcsec and at an average airmass of 1.2. Based on combined images with 22 min total exposure in g'r'i'z' and 25 min in JHK, at a mid-time of 23:55 UT on 2021-09-05 for the afterglow (Strausbaugh & Cucchiara GCN 30769 and 30770; Tanvir et al., GCN 30771; D'Avanzo et al., GCN 30772; Dobie et al., GCN 30773) we derive the following preliminary AB magnitudes and 3-sigma upper limits: g' > 24.3, r' > 24.3, i' > 23.8, z' = 21.6 +/- 0.2, J = 20.2 +/- 0.2, H = 20.1 +/- 0.2, K > 18.2. Given magnitudes are calibrated against the SkyMapper catalog (Wolf et al., 2018, PASA 35, e010) in g'r'i'z' and 2MASS field stars in JHK (Skrutskie et al., 2006, AJ 131, 1163). We thank Maren Hempel and Regis Lachaume for excellent support and for performing the observations.