Modular invariants from subfactors:: Type I coupling matrices and intermediate subfactors

A braided subfactor determines a coupling matrix Z which commutes with the S- and T-matrices arising From the braiding. Such a coupling matrix is not necessarily of type I, i.e. in general it does not have a block-diagonal structure which can be reinterpreted as the diagonal coupling matrix with respect to a suitable extension. We show that there are always two intermediate subfactors which correspond to left and right maximal extensions and which determine parent coupling matrices Z(+/-) of type I. Moreover it, is shown that if the intermediate subfactors coincide, so that Z(+) = Z(-), then Z is related to Z(+) by an automorphism of the extended fusion rules. The intertwining relations of chiral branching coefficients between original and extended S- and T-matrices are also clarified. None of our results depends on non-degeneracy of the braiding, i.e. the S- and T-matrices need not be modular. Examples from SO(n) current algebra models illustrate that the parents can be different, Z(+) not equal Z(-), and that Z need not be related to a type I invariant by such an automorphism.