TITLE: GCN CIRCULAR NUMBER: 13628 SUBJECT: GRB 120811C: Redshift from OSIRIS/GTC DATE: 12/08/11 23:14:09 GMT FROM: Antonio de Ugarte Postigo at IAA-CSIC C. C. Thoene (IAA-CSIC), A. de Ugarte Postigo (IAA-CSIC, DARK), J. Gorosabel, R. Sanchez-Ramirez (IAA-CSIC) and J. P. U. Fynbo (DARK) G. Gomez Velarde (GTC) report on behalf of a larger collaboration: We observed the afterglow of GRB 120811C (Barlow et al., GCN 13622) with OSIRIS at the 10.4m GTC telescope starting at UT 22:07 (6.5 hr after the burst) for a total exposure time of 2400s. We used grism 1000B covering the wavelength range between 3700 and 7800 Angstrom. Despite Calima, the continuum is clearly detected and the spectrum shows strong Ly alpha and Ly beta absorption as well as a number of absorption lines (SII, SiII, OI, CII, CIV, SiIV, AlII, AlIII, NiII, ZnII and SiII*) at a common redshift of z=2.671.