Non-factorization effects in heavy mesons and determination of |V_{ub}| from inclusive semileptonic B decays

The effects of spectator light quark in decays of heavy mesons are considered, which vanish in the limit of factorization of matrix elements of four-quark operators over the mesons. These effects include the difference of the total widths as well as of the semileptonic decay rates between the D^0 and D_s mesons and also a contribution to inclusive semileptonic decay rates of B^0 and B^\pm into the channel X_u l nu related to determination of the weak mixing parameter V_{ub}. If the observed difference of the lifetimes between D_s and D^0 mesons is attributed to non-factorizable terms, such terms can naturally give rise to a significant difference in inclusive semileptonic decay rates of these mesons, and to a light-flavor dependent contribution to decays B -> X_u l nu. The latter contribution affects mostly the upper part of the inclusive spectrum of the invariant mass of the lepton pair, and may significantly exceed the previously claimed in the literature theoretical uncertainty in determination of |V_{ub}|^2 from that part of the spectrum.