  Following the detection of a long GRB 190919B by INTEGRAL (INTErnational
Gamma-Ray Astrophysics Laboratory), we obtained an optical photometric sequence
of its optical counterpart. The light curve of the optical emission exhibits an
unusually steep rise ~100 s after the initial trigger. This behaviour is not
expected from a 'canonical' GRB optical afterglow. As an explanation, we
propose a scenario consisting of two superimposed flares: an optical flare
originating from the inner engine activity followed by the hydrodynamic peak of
an external shock. The inner-engine nature of the first pulse is supported by a
marginal detection of flux in hard X-rays. The second pulse eventually
concludes in a slow constant decay, which, as we show, follows the closure
relations for a slow cooling plasma expanding into the constant interstellar
medium and can be seen as an optical afterglow sensu stricto.
