  We carry out Monte-Carlo simulations to discuss critical properties of a
classical two-dimensional XY frustrated helimagnet on a square lattice. We find
two successive phase transitions upon the temperature decreasing: the first one
is associated with breaking of a discrete Z_2 symmetry and the second one is of
the Berezinskii-Kosterlitz-Thouless (BKT) type at which the SO(2) symmetry
breaks. Thus, a narrow region exists on the phase diagram between lines of the
Ising and the BKT transitions that corresponds to a chiral spin liquid.
