  The unusual slow carrier relaxation dynamics - observed in femtosecond
pump-probe experiments on high-temperature superconductors and recently also in
a charge-density-wave system - is analyzed in terms of a model for relaxation
of carriers in intra-gap states. The data on YBa_2Cu_3O_7-delta near optimum
doping and K_0.3$MoO_3 are found to be described very well with the model using
a BCS-like gap which closes at T_c. From the analysis of the data we conclude
that a significant intra-gap density of localized states exists in these
materials, which can be clearly distinguished from quasiparticle states by the
time-resolved optical experiments because of the different time- and
temperature-dependences of the photoinduced transmission or reflection.
Localized charges are suggested to be the most likely origin of the intra-gap
states, while the similarity of the response in the two materials appear to
exclude spin and vortex excitations.
