  We study a non linear sigma model $O(N)\otimes O(2)/O(N-2)\otimes O(2)$
describing the phase transition of N-components helimagnets up to two loop
order in $D=2+\epsilon$ dimensions. It is shown that a stable fixed point
exists as soon as $N$ is greater than 3 (or equal). In the N=3 case, the
symmetry of the system is dynamically enlarged at the fixed point to O(4) We
show that the order parameter for Heisenberg helimagnets involves a tensor
representation of $O(4)$. We show that for large $N$ and in the neighborhood of
two dimensions this nonlinear sigma model describes the same critical theory as
the Landau-Ginzburg linear theory. We deduce that there exists a dividing line
$N_c (D)$ in the plane $(N, D)$ separating a first-order region containing the
Heisenberg point at $D=4$ and a second-order region containing the whole $D=2$
axis. We conclude that the phase transition of Heisenberg helimagnets in
dimension 3 is either first order or second order with $O(4)$ exponents
involving a tensor representation or tricritical.
