  A (globally) neutral two-body system is supposed to obey a pair of coupled
Klein-Gordon equations in a constant homogeneous magnetic field. Considering
eigenstates of the pseudomomentum four-vector, we reduce these equations to a
three-dimensional eigenvalue problem. The frame adapted to pseudomomentum has
in general a nonvanishing velocity with respect to the frames where the field
is purely magnetic. This velocity plays a crucial role in the occurance of
motional terms; these terms are taken into account within a manifestly
covariant framework. Perturbation theory is available when the mutual
interaction doesnot depend on the total energy; a weak-field-slow-motion
approximation is more specially tractable.
