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The isotopic ratio of nitrogen presents a wide range of values in the SolarSystem and in star forming system whose origin is still unclear. Chemicalreactions in the gas phase are one of the possible processes that could modifythe $^{14}$N/$^{15}$N ratio. We aim at investigating if and how the passage ofa shock wave in the interstellar medium, can affect the relative fraction ofnitrogen isotopes. The ideal place for such a study is the L1157 outflow, whereseveral shocked clumps are present. We present the first measurement of the$^{14}$N/$^{15}$N ratio in the two shocked clumps, B1 and B0, of theprotostellar outflow L1157, derived from the interferomteric maps of theH$^{13}$CN(1-0) and the HC$^{15}$N(1-0) lines. In B1, we find that theH$^{13}$CN(1-0) and HC$^{15}$N(1-0) emission traces the front of the clump,with averaged column density of $N$(H$^{13}$CN) $\sim$ 7$\times$10$^{12}$cm$^{-2}$ and $N$(HC$^{15}$N) $\sim$ 2$\times$10$^{12}$ cm$^{-2}$. In thisregion the ratio H$^{13}$CN(1-0)/HC$^{15}$N(1-0) is quite uniform with anaverage value of $\sim$ 5$\pm$1. The same average value is also measured in thesmaller clump B0e. Assuming the standard $^{12}$C/$^{13}$C = 68, we obtain$^{14}$N/$^{15}$N = 340$\pm$70, similar to those usually found in prestellarcores and protostars. We analysed the prediction of a chemical shock model forseveral shock conditions and we found that the nitrogen and carbonfractionations do not vary much for the first period after the shock. Theobserved H$^{13}$CN/HC$^{15}$N can be reproduced by a non-dissociative, C-typeshock with parameters in agreement with previous modelling of L1157-B1. Bothobservations and chemical models indicate that the shock propagation does notaffect the nitrogen isotopic ratio that remains similar to that measured inlower temperature gas in prestellar cores and in protostellar envelopes.

Seed of Life in Space (SOLIS) XI. First measurement of nitrogen fractionation in shocked clumps of the L1157 protostellar outflow