After orientifold projection, the conifold singularity in hypermultiplet moduli space of Calabi-Yau compactifications cannot be avoided by geometric deformations. We study the fate of this singularity in a local model involving O6-planes and D6-branes wrapping the deformed conifold. After classifying possible orientifolds, we explore the possible geometric transitions by lifting to M-theory as well as by looking at the superpotential. We find a rich pattern of smooth and phase transitions depending on the total sixbrane charge. Non-BPS states from branes wrapped on non-supersymmetric bolts are responsible for a phase transition. Along the way, we obtain a new metric of $G_2$ holonomy corresponding to an O6-plane on the three sphere of the deformed conifold.

SPhT, CEA/Saclay