  Using Monte Carlo simulations, we study thermal and critical properties of
two systems, in which domain walls and so-called $Z_2$-vortices as topological
defects are presented. The main model is a lattice version of the $O(3)$
principal chiral model. We find a first order transition and give qualitative
arguments that the first order is induced by topological defects. We also
consider the model of frustrated antiferromagnet on a square lattice with the
additional exchange interaction between spins of the third range order. This
model belongs to the same symmetry class. In this model, a transition is of
first order too.
