Orientifold Planes, Type I Wilson Lines and Non-BPS D-branes

There is a longstanding puzzle concerned with the existence of Õp − planes with p ≥ 6, which are orientifold p-planes of negative charge with stuck Dp-branes. We study the consistency of configurations with various orientifold planes and propose a resolution to this puzzle. It is argued that Õ6 − -planes are possible in massive IIA theory with odd cosmological constant, while Õ7 − -planes and Õ8 − -planes are not allowed. Various relations between orientifold planes and non-BPS D-branes are also addressed. E-mail : [email protected] E-mail : [email protected] Address from July 2000 : Detartment of Physics, University of Tokyo, Hongo, Tokyo 113-0033, Japan E-mail : [email protected]