  The unbinding of kink pairs on domain walls in the fully frustrated XY model
(on square or triangular lattices) is shown to induce the vanishing of phase
coupling across the walls. This forces the phase transition, associated with
unbinding of vortex pairs to take place at a lower temperature than the other
phase transition, associated with proliferation of the Ising-type domain walls.
The results are applicable for a description of superconducting junction arrays
and wire networks in perpendicular magnetic field (corresponding to
half-integer number of flux quanta per each lattice plaquette), as well as of
planar antiferromagnets with a triangular lattice.
