Komech A., Kunze M., Spohn H.=20 Effective dynamics for a mechanical particle coupled to a wave field (57K, LATeX 2e) ABSTRACT. We consider a particle coupled to a scalar wave field and subject to the slowly varying potential. We prove that if the initial state is close to a soliton (=3Ddressed particle), then the solution stays forever close to t= he soliton manifold. This estimate implies that over a the approriate time span the radiation losses are negligible and that the motion of the particle is governed by an effective Hamiltonian.