Electric dipole moment of the neutron in chiral perturbation theory

We calculate the electric dipole moments of the neutron and the $\Lambda$ within the framework of heavy baryon chiral perturbation theory. They are induced by strong $CP-\mathrm{violating}$ terms of the effective Lagrangian in the presence of the vacuum angle ${\theta }_0.$ The construction of such a Lagrangian is outlined and we are able to give an estimate for ${\theta }_0.$