  We consider gauged Wess-Zumino models based on the non compact group
$SU(2,1)$. It is shown that by vector gauging the maximal compact subgroup
$U(2)$ the resulting backgrounds obey the gravity-dilaton one loop string
vacuum equations of motion in four dimensional euclidean space. The torsionless
solution is then interpreted as a pseudo-instanton of the $d=4$ Liouville
theory coupled to gravity. The presence of a traslational isometry in the model
allows to get another string vacuum backgrounds by using target duality that we
identify with those corresponding to the axial gauging. We also compute the
exact backgrounds. Depending on the value of $k$, they may be interpreted as
instantons connecting a highly singular big bang like universe with a static
singular or regular black plane geometry.
