TITLE: GCN CIRCULAR NUMBER: 18274 SUBJECT: GRB 150910A: Redshift from 10.4m GTC+OSIRIS DATE: 15/09/11 00:17:10 GMT FROM: Antonio de Ugarte Postigo at IAA-CSIC Antonio de Ugarte Postigo (IAA-CSIC, DARK/NBI), Christina Thoene (IAA-CSIC), Gianluca Lombardi (IAC, ULL, GRANTECAN), and Alberto Perez (GRANTECAN) report on behalf of a larger collaboration: We have observed the afterglow of GRB 150910A (Pagani et al., GCN 18264) using the 10.4 m GTC telescope equipped with OSIRIS. Our observation began at 23:11 UT (14.1 hr after the burst) and consisted of 3x900s exposures using the R1000B grism. The spectrum covers the wavelength range between 3700 and 7800 AA. The acquisition image shows the afterglow at a magnitude of r=20.65+/-0.07 as compared to SDSS stars. The first spectrum shows a continuum throughout the full spectral range, with several absorption features that correspond to AlII, AlIII, FeII, MgII and MgI at a common redshift of 1.360, which we interpret as the redshift of the GRB.