  We investigate a solution of the Weyl invariance conditions in open string
theory in 4 dimensions. In the closed string sector this solution is a
combination of the SU(2) Wess--Zumino--Witten model and a Liouville theory. The
investigation is carried out in the $\sigma$ model approach where we have
coupled all massless modes (especially an abelian gauge field via the boundary)
and tachyon fields. Neglecting all higher derivatives in the field strength we
get an exact result which can be interpreted as a monopole configuration living
in non--trivial space time. The masses of both tachyon fields are quantized by
$c_{wzw}$ and vanish for $c_{wzw} = 1$.
