TITLE: GCN CIRCULAR NUMBER: 26701 SUBJECT: LIGO/Virgo S200105ae: AT2020oo 10.4m GTC spectroscopy DATE: 20/01/11 09:47:25 GMT FROM: Alberto J. Castro-Tirado at IAA-CSIC Y.-D. Hu (IAA-CSIC), A. F. Valeev (SAO-RAS), A. J. Castro-Tirado and E. Fernandez-Garcia (IAA-CSIC), V. Sokolov (SAO-RAS), I. Carrasco (UMA) and M. D. Caballero Garcia (ASU-CAS) and J. Font (GRANTECAN, IAC, ULL), on behalf of a larger collaboration, report: Following the detection of AT2020oo/Gaia20add (Kostrzewa-Rutkowska et al. GCNC 26686) within the error area of the GW event S200105ae (LVC, GCNC 26640), we obtained an optical spectrum covering the range 3700-7400 A with the 10.4m GTC telescope equipped with OSIRIS in La Palma (Spain) on Jan 10, 06:53 UT. For AT2020oo/Gaia20add, a magnitude r'= 17.56 +/- 0.02 at 06:49 UT is derived. The GTC spectrum is consistent with a SN Ia at redshift z = 0.0486 +/- 0.0005 (based on the emission lines of the host galaxy). Therefore we consider that AT2020oo/Gaia20add is unrelated to the S200105ae GW alert.