  We construct a new class of exact string solutions with a four dimensional
target space metric of signature ($-,+,+,+$) by gauging the independent left
and right nilpotent subgroups with `null' generators of WZNW models for rank 2
non-compact groups $G$. The `null' property of the generators (${\rm Tr }(N_n
N_m)=0$) implies the consistency of the gauging and the absence of
$\a'$-corrections to the semiclassical backgrounds obtained from the gauged
WZNW models. In the case of the maximally non-compact groups ($G= SL(3),
SO(2,2), SO(2,3), G_2$) the construction corresponds to gauging some of the
subgroups generated by the nilpotent `step' operators in the Gauss
decomposition. The rank 2 case is a particular example of a general
construction leading to conformal backgrounds with one time-like direction. The
conformal theories obtained by integrating out the gauge field can be
considered as sigma model analogs of Toda models (their classical equations of
motion are equivalent to Toda model equations). The procedure of `null gauging'
applies also to other non-compact groups.
