GCN Circular 20245
���Z. Cano (IAA-CSIC), D. Malesani (DARK/NBI and DTU Space), A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), L. Izzo, C. C. Thoene (IAA-CSIC) and N. Castro-Rodriguez (GRANTECAN/IAC/Universidad de La Laguna) report: We observed the field of GRB 161129A (Kocevski et al., GCN Circ. 20210) with OSIRIS on the 10.4 m GTC telescope on La Palma (Spain). The observations started at 19:46:40.9 UT on 05-December-2016 (6.52 days after the burst), which consisted of 4 x 1200 s spectra using grism R1000B, which covers the wavelength range 3700-7800 angstroms, with a resolution of R~1000. Within the enhanced XRT error region (Beardmore et al. 2016, GCN Circ. 20211) we note the presence of sources A & B detected by Heintz et al. (GCN Circ. 20244). The spectrum of the extended object (source B) shows a continuum with a strong emission line that we attribute to the [OII] 3727/3729 doublet at z=0.645, as well as weaker emission lines consistent with being H-gamma and H-delta at the same redshift. We propose this star-forming galaxy to be the host of GRB 161129A. The spectrum of source A displays absorption features of Ca II K and H, Mg I and MgH at z~0, confirming it is a foreground star and not related to GRB 161129A.