TITLE: GCN CIRCULAR NUMBER: 15167 SUBJECT: GRB 130831A: Montarrenti Observatory optical follow-up DATE: 13/09/02 12:43:02 GMT FROM: Simone Leonini at Monarrenti Obs Simone Leonini, G. Guerrini, P. Rosi and L.M. Tinjaca Ramirez (Montarrenti Observatory, Siena, Italy) report: We continue to observe and detect the fading optical afterglow of GRB 130831A (Swift trigger 568849, Hagen et al., GCN Circ. 15139) with the automatic 0.53m RC telescope + U47 detector at Montarrenti Observatory (Siena, Italy, IAU code C88). We stacked 4 unfiltered CCD exposures of 60s each and we detect the very faint OT at the same position previously measured (GCN 15150) obtaining the following photometry at t~34h : 2013 Sep. 01.95733 UT R=20.46 +/- 0.24 Magnitudes were obtained using Astrometrica software using USNO-B1 catalogue for positions and photometry. An ensemble of 66 stars were used for the magnitude comparison.