  We report on a two-flavour lattice QCD study of the $B_s \to D_s$ and $B_s
\to D^*_s$ transitions parameterized, in the heavy quark limit, by the form
factors ${\cal G}$, and $h_{A_1}$, $h_{A_2}$ and $h_{A_3}$, respectively. In
the search for New Physics through tests of lepton flavour universality, $B_s$
decay channels are complementary to the $B$ decays widely studied at $B$
factories and LHCb, while on the theory side they can be better controlled than
the $B_c$ and $\Lambda_b$ decays. The purpose of this exploratory two-flavour
study is, in preparation for future analyses of lattice QCD simulations with
${N_f>2}$ and physical quark-masses, to gain experience on a suitable method
for a lattice extraction of form factors associated with $b \to c$ currents
that may yield tighter control over systematic effects like contamination from
excited states and cut-off effects. We obtain the zero-recoil values ${\cal
G}^{B_s \to D_s}(1)=1.03(14)$ and $h^{B_s \to D^*_s}(1)=0.85(16)$.
