TITLE: GCN CIRCULAR NUMBER: 10488 SUBJECT: GRB 100316 optical candidate from 1.23m CAHA telescope DATE: 10/03/16 05:16:37 GMT FROM: Javier Gorosabel at IAA-CSIC J. Gorosabel (IAA-CSIC), V. Peris (U. Valencia), P. Kubanek (IAA-CSIC, U. Valencia), report on behalf of a larger collaboration: "We have carried out R-band observations of the GRB100316 BAT error box (Baumgartner et al., GCN 10484) with the 1.23m telescope of Calar Alto. The observations started on Feb 16.1075 UT (11.7 min after the GRB). We detect an object with a rough magnitude of R~20.5 coincident with the XRT (GCN 10485) position and very likely not present on the DSS. Further observations are encouraged to determine the possible fading behaviour of this source. We propose that object as the potential optical afterglow of GRB100316."