  GRB 990413 shows a very hard spectrum (with a low energy spectral component
F(E) \propto E^{2.49}) which is well represented by a black body model with
characteristic temperature ~70 keV. It thus belongs to the subset of GRBs which
might be revealing a thermal phase. We find that the temperature/luminosity
evolution is consistent with that found in the other ``thermal'' GRBs. The time
resolved spectral analysis indicates the presence of a second non--thermal
component contributing (for about 1 s) up to 30 per cent of the total flux.
Differently from the other thermal GRBs, GRB 990413 shows significantly high
level of variability and the evolution of the thermal/non--thermal spectral
components is strongly correlated with the flux variations. This GRB thus
offers the unique opportunity to test the standard fireball photospheric and
internal shock phases and their reciprocal influence. GRB 990413 was not
selected on the basis of its spectrum and thus hints to the possibility that
this early behavior might be more common than currently known.
