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The 16 Cyg system is composed of two solar analogs with similar masses andages. A red dwarf is in orbit around 16 Cyg A whereas 16 Cyg B hosts a giantplanet. The abundances of heavy elements are similar in the two stars butlithium is much more depleted in 16 Cyg B that in 16 Cyg A, by a factor of atleast 4.7. The interest of studying the 16 Cyg system is that the two star havethe same age and the same initial composition. The presently observeddifferences must be due to their different evolution, related to the fact thatone of them hosts a planet contrary to the other one. We computed models of thetwo stars which precisely fit the observed seismic frequencies. We used theToulouse Geneva Evolution Code (TGEC) that includes complete atomic diffusion(including radiative accelerations). We compared the predicted surfaceabundances with the spectroscopic observations and confirmed that anothermixing process is needed. We then included the effect of accretion-inducedfingering convection. The accretion of planetary matter does not change themetal abundances but leads to lithium destruction which depends on the accretedmass. A fraction of earth mass is enough to explain the lithium surfaceabundances of 16 Cyg B. We also checked the beryllium abundances. In the caseof accretion of heavy matter onto stellar surfaces, the accreted heavy elementsdo not remain in the outer convective zones but they are mixed downwards byfingering convection induced by the unstable $\mu$-gradient. Depending on theaccreted mass, this mixing process may transport lithium down to its nucleardestruction layers and lead to an extra lithium depletion at the surface. Afraction of earth mass is enough to explain a lithium ratio of 4.7 in the 16Cyg system. In this case beryllium is not destroyed. Such a process may befrequent in planet host stars and should be studied in other cases in thefuture.

Accretion of planetary matter and the lithium problem in the 16 Cygni stellar system