# Orientifold Plane, Anti-Dp Brane and SUSY breaking

In "String Theory and M-Theory" by K. Becker, M. Becker and J.H. Schwarz, page 222, they give a brief introduction about the (space-filling) Orientifold Plane $O9$ as an object needs to be add in the theory to make the it consistence (or at least, that what I thought). However, I still don't really get their arguments.

1. What are the physical properties of the Orientifold Plane $Op$ in String Theory?

2. How are they described in the context of targer space? How do they couple with objects in String theory (string + brane)?

3. Is there any low-effective description for it? What're the theoretical evidences of their existence in String theory?

4. Moreover, they also say that (on the same page) the presence of $\bar{Dp}$ branes breaks all the SUSY. How can I see that?

• Wikipedia : "O-planes and D-branes can be used within the same construction and generally carry opposite tension to one another. However, unlike D-branes, O-planes are not dynamical. They are defined entirely by the action of the involution, not by string boundary conditions as D-branes are. Both O-planes and D-branes must be taken into account when computing tadpole constraints." – Trimok Aug 14 '13 at 18:55