The equations of motion of nonspinning compact binary systems at the third post-Newtonian (PN) order in massless scalar-tensor theories have recently been obtained. In the present paper, we complete this work by computing at 3PN order the ten integrals of motion together with the equations of motion in the center-of-mass frame. We then perform the reduction of the center-of-mass equations to quasi-circular orbits and determine the conserved energy and angular momentum for circular orbits.
I. INTRODUCTIONThe detections of gravitational waves emitted by inspiralling compact binaries is a first step towards a better comprehension of gravitational physics [1]. In the future, the next generation of high precision experiments will allow a better understanding of these systems, by constraining their abundance, origin and parameters. It will also provide information about the strong-field and highly-dynamical regime of gravitational physics, by challenging the theory of general relativity (GR) in this regime.The detection and parameter estimation of gravitational wave events require a bank of highly accurate templates for the gravitational waveforms, to be match-filtered against the data. Current templates are constructed by matching the waveform at different stages of the coalescence, that are obtained using different approaches: post-Newtonian formalism for the inspiral [2] and numerical relativity for the merger and ringdown [3]. There are currently two main series of template waveforms to be used by the LIGO-Virgo collaboration, based either on a direct matching (IMR models) [4,5] or on some resummation techniques (EOB waveforms) [6,7].In order to perform tests of general relativity, one also has to construct such templates for the many alternative theories of gravity. There exist two different and complementary approaches to this problem: either theoryindependent [8] or theory-dependent [9]. Although fundamentally different, these two approaches are complementary as one needs to be able to map the constraints coming from the agnostic approach to specific theories. Here, we will focus on a specific model, namely massless scalar-tensor theory [10,11], one of the oldest, most popular and well-studied theory of gravity [12]. It consists in adding a single massless scalar field, coupled in a minimal way to gravity. One of the motivations for studying this theory, which usually arises as a low-energy limit of some string theory, relies on the fact that it may explain the accelerated expansion of the universe [13].The present work is part of a series of articles whose attempt is to construct the full gravitational and scalar waveforms in scalar-tensor theories up to 2PN order 1 [14-17]. In the companion paper [17], that will be referred to as Paper I in the following, we computed the harmonic coordinate equations of motion in scalar-tensor theories at 3PN order. In the present paper, we complete this work by determining the center-of-mass and conserved quantities to the same 3PN order in the center-of-mass frame. As the le...