TITLE: GCN CIRCULAR NUMBER: 10286 SUBJECT: GRB 091221, GROND observations DATE: 09/12/22 03:20:01 GMT FROM: Sylvio Klose at TLS Tautenburg R. Filgas, P. Afonso (both MPE Garching), S. Klose (TLS Tautenburg), and J. Greiner (MPE Garching), report on behalf of the GROND team: GROND (Greiner et al. 2008, PASP 120, 405), the 7-channel imager mounted at the 2.2m ESO/MPI telescope at La Silla Observatory (Chile), started follow-up observations of GRB 091221 (Krimm et al. 2009, GCN 10283) on Dec 22, at 01:27 UT (4.5 hrs after the burst). For the suggested GRB afterglow (Zheng et al. 2009, GCN 10284; Holland et al. 2009, GCN 10285), based on a first 8 min exposure, we estimate the following preliminary magnitudes (in the AB system): g' = 22.90 +/- 0.30 r' = 22.27 +/- 0.30 i' = 21.95 +/- 0.30 z' = 22.20 +/- 0.30 J = 20.16 +/- 0.20 H > 19.9 K > 19.1 using GROND zeropoints and 2MASS catalog (airmass 1.8). Observations are ongoing. Taking into account a Galactic reddening of E(B-V)=0.22 mag (Holland et al. 2009, GCN 10285), we estimate a photometric redshift of z<3.5. At present we cannot decide if the z'-band magnitude is indicating to a broad-band absorption feature or if we underestimate the magnitude error in this band (where the source is very faint).