We use the M theory approach in the presence of an orientifold O6 plane to
understand some aspects of the moduli space of vacua for N=1 supersymmetric
SO(N-c)/Sp(N-c) gauge theories in four dimensions. By exploiting some gene
ral properties of the O6 orientifold, we reproduce some results obtained pr
eviously with an orientifold O4 plane when the flavor group arises from the
world volume dynamics of D6 branes. By using semi-infinite D4 branes inste
ad of D6 branes, we derive the most general form of the rotated curve descr
ibing the moduli space of vacua for N=1 supersymmetric gauge theory. [S0556
-2821(99)07402-0].