A generalized spin ladder in a magnetic field

We study the phase diagram of coupled spin-1/2 chains with bilinear and (ch iral) three-spin exchange interactions in a magnetic field. The model is so luble on a one-parametric line in the space of coupling constants connectin g the limiting cases of a single and two decoupled Heisenberg chains with n earest neighbour exchange only. We give a complete classification of the lo w-energy properties of the integrable system and introduce a numerical meth od which allows to study the possible phases of spill ladder systems away f rom the soluble line in a magnetic field.