A system of two fermions with different masses and interacting by the Coulomb
potential is presented in a completely covariant framework. The spin-spin
interaction, including the anomalous magnetic moments of the two fermions, is
added by means of a Breit term. We solve the resulting fourth order
differential system by evaluating the spectrum and the eigenfunctions. The
interaction vertex with an external electromagnetic field is then determined.
The relativistic eigenfunctions are used to study the photon emission from a
hyperfine transition and are checked for the calculation of the Lamb shift due
to the electron vacuum polarization in the muonic Hydrogen.Comment: 8 page
