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We report on a detection of an early rising phase of optical afterglow (OA)of a long GRB 090726. We resolve a complicated profile of the optical lightcurve. We also investigate the relation of the optical and X-ray emission ofthis event. We make use of the optical photometry of this OA obtained by the0.5 m telescope of AI AS CR, supplemented by the data obtained by otherobservers, and the X-ray Swift/XRT data.  The optical emission peaked at ~ 17.5 mag (R) at t-T0 ~ 500 s. We find acomplex profile of the light curve during the early phase of this OA: anapproximately power-law rise, a rapid transition to a plateau, a weak flaresuperimposed on the center of this plateau, and a slowly steepening earlydecline followed by a power-law decay. We discuss several possibilities toexplain the short flare on the flat top of the optical light curve at t-T0 ~500 s; activity of the central engine is favored although reverse shock cannotbe ruled out. We show that power-law outflow with Theta_obs/Theta_c > 2.5 isthe best case for OA of GRB 090726. The initial Lorentz factor is Gamma_0 ~230-530 in case of propagation of the blast wave in a homogeneous medium, whilepropagation of this wave in a wind environment gives Gamma_0 ~ 80-300. Thevalue of Gamma_0 in GRB 090726 thus falls into the lower half of the rangeobserved in GRBs and it may even lie on the lower end. We also show that boththe optical and X-ray emission decayed simultaneously and that the spectralprofile from X-ray to the optical band did not vary. This OA belongs to theleast luminous ones in the phase of its power-law decay corresponding to thatobserved for the ensemble of OAs of long GRBs.

Complicated variations of early optical afterglow of GRB 090726