  Two positive mass, spin $\frac 12$ particles created in an entangled state
are studied in the presence of a constant magnetic field inducing distinct
precessions, depending on the respective momenta, of the two spins. The charge
and anomalous magnetic moment of each particle is taken into account.
Consequences for entanglement and, more generally, on correlations, are
derived. We start, however, with a compact derivation of the effects of Lorentz
transformations on such entangled states, though that has been studied by
several authors. Our formalism displays conveniently the analogies and the
differences between the two cases. Moreover, combining the two, one obtains the
case of constant, orthogonal electric and magnetic fields. More general
perspectives are evoked in the concluding remarks.
