  This article devoted to relativistic dynamics of a charged massive particle
in an electroscalar field. It represents a continuation of paper [1] where the
authors constructed a non-relativistic theory which describes transverse
electromagnetic waves along with longitudinal electroscalar ones, responsible
for the wave transport of the Coulomb field. A new type of relativistic force
exerted by electroscalar field on an electrically charged particle and the
relativistic law of superposition of electromagnetic transverse and
electroscalar longitudinal fields are established. Also, a relativistically
invariant form of a Lagrangian describing the interaction between an
electroscalar field and massive electrically charged particle is defined.
