Towards Mass Deformed N=4 SO(n) and Sp(k) gauge configurations
  We study the introduction of orientifold six-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 $\b=0$ models is solved for vanishing sum of hypermultiplet masses by embedding the type IIA configuration into M-theory. We also find 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. 
Angel M. Uranga