  We study the low-energy properties of a Heisenberg spin-1/2 zigzag ladder
with different exchange constants on the two chains. Using a nonlinear
sigma-model field theory and abelian bosonization, we find that the excitations
are gapless, with a finite spin wave velocity, if the values of the chain
exchanges are small. If the chain exchanges are large, the system is gapped,
and the energy spectra of the kink and antikink excitations are different from
each other.
