TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3658 SUBJECT: GRB050721A: optical decay DATE: 05/07/21 08:02:10 GMT FROM: Paolo D'Avanzo at INAF-OAB P. D'Avanzo, S. Covino, (INAF/OABr), D. Malesani (SISSA), S. Bagnulo, T. Szeifert (ESO), G. Tagliaferri (INAF/OABr), G. Chincarini (Univ. Milano- Bicocca), S. Piranomonte, L.A. Antonelli, and L. Stella (INAF/OAR), on behalf of the MISTICI collaboration, report: We performed a preliminary analysis on our R-band images (Covino et al., GCN 3657) of the field of GRB050721 (Antonelli et al., GCN 3654). The afterglow indicated by Torii (GCN 3655) and confirmed by Covino et al. (GCN 3657) faded by ~ 0.9 mag in the R band between July 21.204 UT and July 21.226 UT (25 and 56 minutes after the GRB respectively). This implies a decay index of ~ 1. At our last epoch, the afterglow magnitude was R ~ 18.6, with reference to USNO star UO600_22128314 assumed to be R = 17.4 This message can be cited.