Starting with configurations of fourbranes, fivebranes, sixbranes and
orientifolds in type IIA string theory we derive via M-theory the curv
es solving N = 2 supersymmetric gauge theories with gauge groups SO(N)
and Sp(2N). We also obtain new curves describing theories with produc
t gauge groups. A crucial role in the discussion is played by the inte
raction of the orientifolds with the NS-fivebranes. (C) 1997 Elsevier
Science B.V.