TITLE: GCN CIRCULAR NUMBER: 17198 SUBJECT: GRB 141220A: Redshift from OSIRIS at the 10.4m GTC DATE: 14/12/20 07:53:28 GMT FROM: Antonio de Ugarte Postigo at IAA-CSIC A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), C.C. Thoene (IAA-CSIC), J. Gorosabel (UPV, IAA-CSIC), N. Tanvir (U. Leicester), J.P.U. Fynbo (DARK/NBI), P. Pesev (GRANTECAN), G. Gomez Velarde (GRANTECAN), and D. Perez Valladares (GRANTECAN) report on behalf of a larger collaboration We observed the afterglow of GRB141220A (Cummings et al. GCN 17196, Gorosabel et al. GCN 17197) with OSIRIS at the 10.4m GTC. Observations began at 6:45 UT (50 min after the burst). In a first 600 s spectrum obtained with grism R1000B covering from 3750 to 7800 A we detect absorption features of AlII, FeII, MgII, MgI, at a redshift of 1.3195, including several absorptions of FeII* which make it very likely the redshift of the GRB. We also detect two MgII intervening systems at redshifts of 1.280 and 0.527.