TITLE: GCN CIRCULAR NUMBER: 33691 SUBJECT: GRB 230426A: MASTER OT J023135.16-792104.8 optical counterpart discovery DATE: 23/04/27 17:28:36 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs V.Lipunov (Lomonosov MSU), D.A.H.Buckley (SAAO), Ya.Kechin, A.Chasovnikov, A.Kuznetsov, N.Tiurina, P.Balanutsa, O.Gress, E.Gorbovskoy, G.Antipov, K.Zhirkov, D.Vlasenko, V.Senik, V.Topolev, Yu.Tselik, Siyu Wu (Lomonosov Moscow State University, SAI, Physics Department), C.Francile, F. Podesta, C.Lopez, R. Podesta (Observatorio Astronomico Felix Aguilar OAFA), R. Rebolo, M. Serra (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory), N.M.Budnev, O.Ershova (ISU,API), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A. Gabovich, V.Yurkov (Blagoveschensk Educational State University), K.Vetrov, V.Shumkov, T.Pogrosheva MASTER-SAAO robotic telescope (Global MASTER Net http://observ.pereplet.ru,Lipunov et al., 2010, Advances in Astronomy, v.2010, 30L) located in South African Astronomical Observatory started (Lipunov et al. GCN 33685) inspect of the FERMI GRB230426.85 (trigger 704233744, Wood et al. GCN 33689, Ttrigger 20:28:59.97UT) errorbox 4359 sec after notice time (4391 sec after trigger time) at 2023-04-26 21:42:11UT with upper limit up to 19.5m, see our cover map https://master.sai.msu.ru/site/master2/observ.php?id=2212931 MASTER OT J023135.16-792104.8 - optical counterpart of GRB 230426A discovery 2023gwu MASTER-SAAO auto-detection system found optical counterpart of GRB 230426.85 at R.A.,Dec(2000) = 02h 31m 35.16s , -79d 21m 04s.8 with unfiltered m_OT=17.4 at 2023-04-26 21:56:50UT and with optical decay at 10 images https://www.wis-tns.org/object/2023gwu . There is no any sources in VIZIER database, no any objects in MPC at this time and no OT in MASTER database since Dec.2014. We used reference image on 2018-08-18.99148 UT with  unfiltered mlim=20.6m. Analysis will be continued. Deep photometry and spectral observations are required. The message may be cited.