Towards mass deformed N=4 SO(n) and Sp(k) theories from brane configurations

We study the introduction of orientifold 6-planes in the type IIA brane configurations known as elliptic models. The N= 4 SO(n) and Sp(k) theories softly broken to N = 2 through a mass for the adjoint hypermultiplet can be realized in this framework in the presence of two orientifold planes with opposite RR charge. A large class of beta = 0 models is served for vanishing sum of hypermultiplet masses by embedding the type IIA configuration into M-theory. We also End a geometric interpretation of Montonen-Olive duality based on the properties of the curves. We make a proposal for the introduction of non-vanishing sum of hypermultiplet masses in a sub-class of models. In the presence of two negatively charged orientifold planes and four D6-branes other interesting beta = 0 theories are constructed, e.g. Sp(k) with four flavours and a massive antisymmetric hypermultiplet. We comment on the difficulties in obtaining the curves within our framework due to the arbitrary positions of the D6-branes, (C) 1998 Elsevier Science B.V.