  The Covariant Spectator Theory (CST) is used to calculate the mass spectrum
and vertex functions of heavy-light and heavy mesons in Minkowski space. The
covariant kernel contains Lorentz scalar, pseudoscalar, and vector
contributions. The numerical calculations are performed in momentum space,
where special care is taken to treat the strong singularities present in the
confining kernel. The observed meson spectrum is very well reproduced after
fitting a small number of model parameters. Remarkably, a fit to a few
pseudoscalar meson states only, which are insensitive to spin-orbit and tensor
forces and do not allow to separate the spin-spin from the central interaction,
leads to essentially the same model parameters as a more general fit. This
demonstrates that the covariance of the chosen interaction kernel is
responsible for the very accurate prediction of the spin-dependent
quark-antiquark interactions.
