  We discuss gamma-ray bursts that have very hard spectra, consistent with
black-body radiation, throughout their duration. We find that the temperature
decay during a pulse can be well described by a broken power-law in time, with
an initially constant or weak decay. After the break, most cases are consistent
with a decay with index -2/3. Some pulses have a weak non-thermal component
overlayed the thermal one, and are better modelled with a combination of a
thermal and a non-thermal component. Such a two-component model can explain the
whole time-evolution of other bursts, that are found to be only initially
thermal and later become non-thermal. The relative strengths between the two
components vary with time and this is suggested to, among other things, account
for the change in the modelled low-energy power-law slope that is often
observed in bursts. We interpret the observations within a model of an
optically thick shell that expands adiabatically. The slow, or constant,
temperature decrease is from the acceleration phase, during which the bulk
Lorentz factor increases, and the faster temperature decay is reached as the
flow saturates and starts to coast with a constant speed. We also discuss a
Poynting-flux model, in which the saturation radius is reached close to the
photosphere.
