  We study two Heisenberg spin-1/2 chains coupled by a frustrating ``zigzag''
interaction. We are particularly interested in the regime of weak interchain
coupling, which is difficult to analyse by either numerical or analytical
methods. Previous density matrix renormalisation group (DMRG) studies of the
isotropic model with open boundary conditions and sizeable interchain coupling
have established the presence of incommensurate correlations and of a spectral
gap. By using twisted boundary conditions with arbitrary twist angle, we are
able to determine the incommensurabilities both in the isotropic case and in
the presence of an exchange anisotropy by means of exact diagonalisation of
relatively short finite chains of up to 24 sites. Using twisted boundary
conditions results in a very smooth dependence of the incommensurabilities on
system size, which makes the extrapolation to infinite systems significantly
easier than for open or periodic chains.
