  In the framework of three-point QCD sum rules, the form factors for the
semileptonic decays of B_c --> B_s(B_s^*) l \nu are calculated with account for
the Coulomb-like alpha_s/v-corrections in the heavy quarkonium. The generalized
relations due to the spin symmetry of HQET/NRQCD for the form factors are
derived at the recoil momentum close to zero. The nonleptonic decays are
studied using the assumption on the factorization. The B_c meson lifetime is
estimated by summing up the dominating exclusive modes in the c --> s
transition combining the current calculations with the previous analysis of b
--> c decays in the sum rules of QCD and NRQCD.
