  The effect of a magnetic field on the equilibrium spectral and transport
properties of a single-molecule junction is studied using the numerical
renormalization group method. The molecule is described by the
Anderson-Holstein model in which a single vibrational mode is coupled to the
electron density. The effect of an applied magnetic field on the conductance in
the Kondo regime is qualitatively different in the weak and strong
electron-phonon coupling regimes. In the former case, the Kondo resonance is
split and the conductance is strongly suppressed by a magnetic field $g mu_B B
\gtrsim k_BT_K$, with $T_K$ the Kondo temperature. In the strong
electron-phonon coupling regime a charge analog of the Kondo effect develops.
In this case the Kondo resonance is not split by the field and the conductance
in the Kondo regime is enhanced in a broad range of values of $B$.
