  We study 7 Gamma Ray Bursts (GRBs), detected both by the BATSE instrument,
on-board the Compton Gamma Ray Observatory, and by the Wide Field Camera (WFC),
on-board BeppoSAX. These bursts have measured spectroscopic redshifts and are a
sizeable fraction of the bursts defining the correlation between the peak
energy E_peak (i.e. the peak of the vFv spectrum) and the total prompt
isotropic energy E_iso (the so called "Amati" relation). Recent theoretical
interpretations of this correlation assume that black-body emission dominates
the time resolved spectra of GRBs, even if, in the time integrated spectrum,
its presence may be hidden by the change of its temperature and by the dilution
of a possible non-thermal power law component. We perform a time resolved
spectral analysis, and show that the sum of a power-law and a black-body gives
acceptable fits to the time dependent spectra within the BATSE energy range,
but overpredicts the flux in the WFC X-ray range. Moreover, a fit with a cutoff
power-law plus a black-body is consistent with the WFC data, but the black-body
component contributes a negligible fraction of the total flux. On the contrary,
we find that fitting the spectra with a Band model or a simple cutoff power-law
model yields an X-ray flux and spectral slope which well matches the WFC
spectra.
