The representation in terms of Ernst's complex potential is used to describe
and analyze dilaton-axion theories with potential. The set of such systems is
divided into pairs of dual systems with respect to the inversion of the Ernst
potential. Using duality, a theory is constructed that is invariant with
respect to the nonlinear Ehlers transformation. For this theory, a soliton
solution is obtained that is dual to a dilaton kink in a system that is
invariant with respect to the axion shift transformation.Comment: 9 page