  Starting from the second post-Keplerian (2PK) Hamiltonian describing the
conservative part of the two-body dynamics in massless scalar-tensor (ST)
theories, we build an effective-one-body (EOB) Hamiltonian which is a
$\nu$-deformation (where $\nu= 0$ is the test mass limit) of the analytically
known ST Hamiltonian of a test particle. This ST-EOB Hamiltonian leads to a
simple (yet canonically equivalent) formulation of the conservative 2PK
two-body problem, but also defines a resummation of the dynamics which is
well-suited to ST regimes that depart strongly from general relativity (GR) and
which may provide information on the strong-field dynamics, in particular, the
ST innermost stable circular orbit (ISCO) location and associated orbital
frequency. Results will be compared and contrasted with those deduced from the
ST-deformation of the (5PN) GR-EOB Hamiltonian previoulsy obtained in [Phys.
Rev. D95, 124054 (2017)].
