# Two-body problem in Scalar-Tensor theories as a deformation of General Relativity : an Effective-One-Body approach

## TL;DR

This paper develops an Effective-One-Body framework for Scalar-Tensor theories to model the two-body problem, extending general relativity results and enabling analysis of strong-field dynamics.

## Contribution

It introduces a Scalar-Tensor deformation of the EOB Hamiltonian based on the 2PK Hamiltonian, incorporating scalar corrections into the relativistic two-body dynamics.

## Key findings

- Derived the 2PK Hamiltonian for Scalar-Tensor theories.
- Constructed a Scalar-Tensor EOB Hamiltonian via canonical transformation.
- Provides a resummed model for strong-field regime analysis.

## Abstract

In this paper we address the two-body problem in massless Scalar-Tensor (ST) theories within an Effective-One-Body (EOB) framework. We focus on the first building block of the EOB approach, that is, mapping the conservative part of the two-body dynamics onto the geodesic motion of a test particle in an effective external metric. To this end, we first deduce the second post-Keplerian (2PK) Hamiltonian of the two-body problem from the known 2PK Lagrangian. We then build, by means of a canonical transformation a ST-deformation of the general relativistic EOB Hamiltonian which allows to incorporate the Scalar-Tensor (2PK) corrections to the currently best available General Relativity EOB results. This EOB-ST Hamiltonian defines a resummation of the dynamics that may provide information on the strong-field regime, in particular, the ISCO location and associated orbital frequency and can be compared to other, e.g. tidal, corrections.