  We examine the effect of isospin-violating meson-nucleon coupling constants
and of $\pi$-$\eta$ mixing on the binding-energy differences of mirror nuclei
in a model that possesses no contribution from $\rho$-$\omega$ mixing. The
${}^{3}$He-${}^{3}$H binding-energy difference is computed in a nonrelativistic
approach using a realistic wave function. We find the ${}^{3}$He-${}^{3}$H
binding-energy difference very sensitive to the short-distance behavior of the
nucleon-nucleon potential. We conclude that for the typically hard Bonn form
factors such models can not account for the observed binding-energy difference
in the three-nucleon system. For the medium-mass region (A=15--41) the
binding-energy differences of mirror nuclei are computed using a relativistic
mean-field approximation to the Walecka model. We obtain large binding-energy
differences---of the order of several hundred keV---arising from the
pseudoscalar sector. Two effects are primarily responsible for this new
finding: a) the inclusion of isospin breaking in the pion-nucleon coupling
constant, and b) the in-medium enhancement of the small components of the
bound-state wave functions. We look for off-shell ambiguities in these results
and find them to be large.
