  We investigate the effect of resonant temporal forcing on an anisotropic
system that exhibits a Hopf bifurcation to obliquely traveling waves in the
absence of this forcing. We find that the forcing can excite various
phase-locked standing-wave structures: rolls, rectangles and cross rolls. At
onset, at most one of the two - rolls or rectangles - is stable. The cross
rolls can arise in a secondary bifurcation and can be stable. Experimentally,
they would appear as a periodic switching between a structure in which the
`zig'-component dominates and one with dominating `zag'-structure. Since there
are two symmetry-related states of this kind one may expect disordered
structures to arise due to the break-up of the pattern into domains. The
results are consistent with recent experiments on electro-convection in nematic
liquid crystals by de la Torre and Rehberg. We also apply the general analysis
to a model of the behavior near a Lifshitz point, where the angle of
obliqueness vanishes. This analysis indicates that phase-locked standing
rectangles are always unstable in this parameter regime.
