TITLE: GCN CIRCULAR NUMBER: 8540 SUBJECT: GRB 081121: GROND observations DATE: 08/11/22 03:02:14 GMT FROM: Sebastian Loew at MPE S. Loew, T. Kruehler and J. Greiner (all MPE Garching) report on behalf of the GROND team: We observed the field of GRB 081121 (Swift trigger #335105; Oates et al., GCN #8537) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m ESO/MPI telescope at La Silla Observatory (Chile). Observations started at 00:30 UT on November 22nd, 2008, 3.9 hr after the burst at a high airmass of 2.35, and are continuing. We detect the optical afterglow reported by Yuan et al. (GCN #8536) and Oates & Schady (GCN #8538) clearly in all filters. At a midtime of 3.9454 hours after the burst, we estimate the following preliminary magnitudes (all in AB system) in stacked images of 4 x 35s integration in g'r'i'z' and 24 x 10s integration in JHK: g' = 19.1 r' = 18.8 i' = 18.6 z' = 18.5 J = 18.4 H = 18.2 K = 18.1 with typical errors of 0.2 mag, which are mostly due to a preliminary calibration using GROND zeropoints in g'r'i'z' and 2MASS field stars in JHK.