TITLE: GCN CIRCULAR NUMBER: 10402 SUBJECT: GRB 100212A: candidate optical afterglow DATE: 10/02/12 21:32:58 GMT FROM: Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst D. Malesani, G. Leloudas, J. P. U. Fynbo (DARK/NBI), D. Xu (WIS and DARK/NBI), P. Jakobsson (Univ. Iceland), A. Somero, S. Katajainen (Tuorla Obs.), T. Pursimo (NOT), report on behalf of a larger collaboration: We observed the field of GRB 100212A (Grupe et al., GCN 10401) with the NOT equipped with ALFOSC. Observations started on Feb 12.841 UT (6.05 hr after the GRB), in the R band. Close to the refined XRT error circle (as reported in the SPER web page http://www.swift.ac.uk/sper/index.php at 21:30 UT), we detect a source which is not visible in the DSS. Its coordinates are (J2000): RA = 23:45:40.23 Dec = +49:29:40.7 The object is 2.5" away from the centre of the XRT position, currently reported to have a 2.2" error radius. The R-band magnitude of the source is R = 20.3 assuming R = 17.3 for the USNO star at RA = 23:45:42.93, Dec = +49:30:29.3. The source looks pointlike under a seeing of 1" and is clearly brighter than other closeby objects visible in the DSS. We suggest this object to be the afterglow of GRB 100212A.