The torus and the Klein bottle amplitude of permutation orbifolds

The torus and the Klein bottle amplitude coefficients an computed in permut ation orbifolds of RCFT's in terms of the same quantities in the original t heory and the twist group. An explicit expression is presented for the numb er of self conjugate primaries in the orbifold as a polynomial of the total number of primaries and the number of self conjugate ones in the parent th eory. The formulae in the Z(2) orbifold illustrate the general results. (C) 2000 Elsevier Science B.V. All rights reserved.