  Semi-leptonic $B_s \to K \ell \nu$ and $B_s \to D_s \ell \nu$ decays provide
an alternative $b$-decay channel to determine the CKM matrix elements
$|V_{ub}|$ and $|V_{cb}|$ or to obtain $R$-ratios to investigate lepton flavor
universality violations. In addition, these decays may shed further light on
the discrepancies seen in the analysis of inclusive vs. exclusive decays. Using
the nonperturbative methods of lattice QCD, theoretical results are obtained
with good precision and full control over systematic uncertainties. This talk
will highlight ongoing efforts of the $B$-physics program by the RBC-UKQCD
collaboration.
