  An effective sigma model describing behavior of the 3d rigid string with a
$\theta$-term at $\theta=\pi$ is proposed. It contains non-perturbative
corrections resulting from summation over different genera of the 2d surfaces.
The effective theory is the SU(2) WZW model coupled to the Nambu-Goto action.
RG analysis shows the existence of a IR fixed point at which the normal to the
surface has long range correlations. A similar model can describe critical
behaviour of the 3d Y-M fields or the Ising model.
