  We investigate the effect of a magnetic field on the global oscillation modes
of a rotating fluid star in the magnetohydrodynamic approximation. We present
general equations for the modification of any type of fluid mode due to a
general magnetic field which is not aligned with the star's spin axis. In the
case of any internal dipole magnetic field we derive the equations for the
frequency corrections to the r-modes. We solve for the frequency correction
explicitly for the case when the internal dipole field is force-free, including
the uniform density case. In the weak-field limit, the spatial form of the
r-mode velocity perturbation is unchanged, but the magnitude of the frequency
in the rotating frame increases.
