Unstable Non-BPS D-Branes of Type-II String Theories in Light-Cone Green-Schwarz Formalism

The problem of describing the boundary states of unstable non-BPS D-branes of type-II string theories in light-cone Green-Schwarz (GS) formalism is addressed. Regarding the type II theories in light-cone gauge as different realizations of the $\hat{SO}(8)_{k=1}$ Kac-Moody algebra, the non-BPS D-brane boundary states of these theories are given in terms of the relevant Ishibashi states constructed in this current algebra. Using the expressions for the current modes in terms of the GS variables it is straightforward to reexress the boundary states in the GS formalism. The problem that remains is the lack of manifest SO(8) covariance in these expressions. We also derive the various known expressions for the BPS and non-BPS D-brane boundary states by starting with the current algebra Ishibashi states.