  We study a model of two weakly coupled isotropic spin-1/2 Heisenberg chains
with an antiferromagnetic coupling along the chains. It is shown that the
system always has a spectral gap. For the case of identical chains the model in
the continuous limit is equivalent to 4 decoupled noncritical Ising models. For
this case we obtain the exact expressions for the asymptotics of spin-spin
correlation functions. When the chains have different exchange integrals the
spectrum at low energies is well described by the O(3) nonlinear sigma model.
We discuss the topological order parameter related to the gap formation and
give a detailed description of the dynamical magnetic susceptibility.
