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HCN and its isomer HNC play an important role in molecular cloud chemistryand the formation of more complex molecules. We investigate here the impact ofprotostellar shocks on the HCN and HNC abundances from high-sensitivity IRAM30m observations of the prototypical shock region L1157-B1 and the envelope ofthe associated Class 0 protostar, as a proxy for the pre-shock gas. Theisotopologues H$^{12}$CN, HN$^{12}$C, H$^{13}$CN, HN$^{13}$C, HC$^{15}$N,H$^{15}$NC, DCN and DNC were all detected towards both regions. Abundances andexcitation conditions were obtained from radiative transfer analysis ofmolecular line emission under the assumption of Local ThermodynamicalEquilibrium. In the pre-shock gas, the abundances of the HCN and HNCisotopologues are similar to those encountered in dark clouds, with a HCN/HNCabundance ratio $\approx 1$ for all isotopologues. A strong D-enrichment(D/H$\approx 0.06$) is measured in the pre-shock gas. There is no evidence of$^{15}$N fractionation neither in the quiescent nor in the shocked gas. At thepassage of the shock, the HCN and HNC abundances increase in the gas phase indifferent manners so that the HCN/HNC relative abundance ratio increases by afactor 20. The gas-grain chemical and shock model UCLCHEM allows us toreproduce the observed trends for a C-type shock with pre-shock density $n$(H)=$10^5$cm$^{-3}$ and shock velocity $V_s= 40$km/s. We conclude that the HCN/HNCvariations across the shock are mainly caused by the sputtering of the grainmantle material in relation with the history of the grain ices.

HCN/HNC chemistry in shocks: a study of L1157-B1 with ASAI