Ghost D-branes

Tadashi and Takuya have an interesting proposal to interpret the "fermionic" Chan-Paton factors - those that extend a gauge group from U(N) to a supergroup U(N/M) - as creating "ghost D-branes". All their states violate the spin-statistics relation. Note that the cancellation between these two D-branes may be viewed as perfect in topological string theory but Tadashi and Takuya talk about the full string theory.




When a D-brane coincides with a ghost D-brane, the configuration is equivalent to the closed string vacuum. However, when they are separated, they get a new configuration. Using these rules, I am a bit surprised that you can separate them. If the coincident D-brane and ghost D-brane contain no physical open string excitations, does not it mean that we should not be allowed to give one of them a vev?

They also discuss the heterotic dual. In this case, they extend the fermionic formulation by adding new chiral bosons to the 32 fermions.