TITLE: GCN CIRCULAR NUMBER: 19651 SUBJECT: GRB 160625B: RTT150 optical observations DATE: 16/07/04 13:42:20 GMT FROM: Rodion Burenin at IKI, Moscow I. Bikmaev, E. Irtuganov, N. Sakhibullin (KFU/AST), R. Burenin, M. Pavlinsky, R. Sunyaev (IKI), I. Khamitov, H. Kirbiyik (TUG), U. Kiziloglu (METU), E. Gogus (Sabanci Uni.) report: We observed field of Fermi GRB 160625B (Dirirsa et al. GCN 19580) at the position of optical afterglow (Troja et al. ,GCN 19588) with Russian-Turkish 1.5-m telescope (RTT150, Bakirlitepe, TUBITAK National Observatory, Turkey) using the TFOSC instrument. We obtained series of Rc exposures in June 30 / July 01(6 frames x 600 sec, UT = 00:21 - 01:30 ), in July 01 / 02 (4 frames x 900 sec, UT = 00:36 - 01:42), in July 02 / 03 (6 frames x 900 sec, UT = 23:49 - 01:29), and in July 03 / 04, 2016, (14 frames x 600 sec, UT = 22:50 - 01:39). The optical transient (Troja et al. GCN 19588; Oates et al. GCN 19589; Kuroda et al. GCN 19599; Xu et al. GCN 19600; D'Elia et al. GCN 19601; Watson at al. GCN 19602; Karpov et al. GCN 19603; Mazaeva et al. GCN 19605, 19620; Moskvitin et al. GCN 19611; Gorbovskoy et al. GCN 19612; Batsch et al. GCN 19615; Cobb et al. GCN 19616; Guidorzi et al. GCN 19640; Valeev et al. GCN 19642) is clearly detected in each images. Using USNO-B1 0969-0608410 star at RA,DEC = 308.580912 +06.909881 (J2000.0) with R = 16.73 mag as a reference we estimated the Rc-magnitudes of the afterglow in stacked images: T-T0, Mag., Mag.err days 5.09 20.69 +- 0.03 6.10 20.96 +- 0.04 7.08 21.04 +- 0.03 8.06 21.24 +- 0.04 RTT-150 data show that OT is fading at approximately the same power law rate as it was observed earlier.