  A new method to constrain the cosmological equation of state is proposed by
using combined samples of gamma-ray bursts (GRBs) and supernovae (SNeIa). The
Chevallier-Polarski-Linder parameterization is adopted for the equation of
state in order to find out a realistic approach to achieve the
deceleration/acceleration transition phase of dark energy models. As results,
we find that GRBs, calibrated by SNeIa, could be, at least, good distance
indicators capable of discriminating cosmological models with respect to
$\Lambda$CDM at high redshift. Besides, GRBs+SNeIa combined redshift-distance
diagram puts better in evidence the change of slope around redshift $z\sim 0.5$
which is usually addressed as the "signature" of today observed acceleration.
This feature could be interpreted, in more standard way, by the red sequence in
galaxy clusters.
