  We analyze nonleptonic $B_s$ decays to a charmonium state and a light meson,
induced by the $b \to c {\bar c}s$ transition, which are useful to access the
$B_s$-${\bar B}_s$ mixing phase $\beta_s$. We use generalized factorization and
$SU(3)_F$ symmetry to relate such modes to correspondent $B$ decay channels. We
discuss the feasibility of the measurements in the various channels, stressing
the importance of comparing different determinations of $\beta_s$ in view of
the hints of new physics effects (NP) recently emerged in the $B_s$ sector.
Finally, adopting a general parametrization of NP contributions to the decay
amplitudes, we discuss how to experimentally constrain new physics parameters.
