  Considering about seven years of Fermi-Large Area Telescope (LAT) data, we
present a systematic search for variability possibly related to transitions
between states in redbacks and black widow systems. Transitions are
characterized by sudden and significant changes in the gamma-ray flux that
persist on a timescale much larger than the orbital period. This phenomenology
was already detected in the case of two redback systems, PSR J1023+0038 and PSR
J1227-4853, for which we present here a dedicated study. We show the existence
of only one transition for each of these systems over the past seven years. We
determine their spectra, establishing high-energy cutoffs at a few GeV for the
high gamma-ray state of PSR J1023+0038 and for both states of PSR J1227-4853.
The surveying capability of the Fermi-LAT allows studying whether similar
phenomenology has occurred in other sources. Although we have not found any
hint for a state transition for most of the studied pulsars, we note two
black-widow systems, PSR J2234+0944 and PSR J1446-4701, whose apparent
variability is reminiscent of the transitions in PSR J1023+0038 and PSR
J1227-4853. For the other systems we set limits on potential transitions in
their measured gamma-ray light curves.
