Evolution of lithium surface abundances including a mixing at the bottom of the outer convective zone needed to reproduce the 16 Cygni A abundance in the two models (green solid line and blue dashed line; Sect. 4.3). Evolution of lithium surface abundances including accretion of 0.66 M_⊕ for 16 Cygni B (blue dotted line) at the beginning of the main sequence, but with the same mixing as before (Sect. 5). Abundances determined using observations from King et al. (1997) are represented by black crosses.