  We present here a systematic search for cyanopolyynes in the shock region
L1157-B1 and its associated protostar L1157-mm in the framework of the Large
Program "Astrochemical Surveys At IRAM" (ASAI), dedicated to chemical surveys
of solar-type star forming regions with the IRAM 30m telescope. Observations of
the millimeter windows between 72 and 272 GHz permitted the detection of
HC$_3$N and its $^{13}$C isotopologues, and HC$_5$N (for the first time in a
protostellar shock region). In the shock, analysis of the line profiles shows
that the emission arises from the outflow cavities associated with L1157-B1 and
L1157-B2. Molecular abundances and excitation conditions were obtained from
analysis of the Spectral Line Energy Distributions under the assumption of
Local Thermodynamical Equilibrium or using a radiative transfer code in the
Large Velocity Gradient approximation. Towards L1157mm, the HC$_3$N emission
arises from the cold envelope ($T_{rot}=10$ K) and a higher-excitation region
($T_{rot}$= $31$ K) of smaller extent around the protostar. We did not find any
evidence of $^{13}$C or D fractionation enrichment towards L1157-B1. We obtain
a relative abundance ratio HC$_3$N/HC$_5$N of 3.3 in the shocked gas. We find
an increase by a factor of 30 of the HC$_3$N abundance between the envelope of
L1157-mm and the shock region itself. Altogether, these results are consistent
with a scenario in which the bulk of HC$_3$N was produced by means of gas phase
reactions in the passage of the shock. This scenario is supported by the
predictions of a parametric shock code coupled with the chemical model
UCL_CHEM.
