A Non-perturbative Description of the Gimon-Polchinski Orientifold
  A T-dual version of the Gimon-Polchinski orientifold can be described by a configuration of intersecting Dirichlet seven branes and orientifold seven planes in the classical limit. We study modification of this background due to quantum corrections. It is shown that non-perturbative effects split each orientifold plane into a pair of nearly parallel seven branes. Furthermore, a pair of intersecting orientifold planes, instead of giving rise to two pairs of intersecting seven branes, gives just one pair of seven branes, each representing a pair of nearly orthogonal seven branes smoothly joined to each other near the would be intersection point. Interpretation of these results from the point of view of the dynamics on a three brane probe is also discussed. 
Ashoke Sen