T-duality in M-theory and supermembranes

The (q₁, q₂) SL(2, Z) string bound states of type IIB superstring theory admit two inequivalent (T-dual) representations in eleven dimensions in terms of a fundamental 2-brane. In both cases, the spectrum of membrane oscillations can be determined exactly in the limit g² -> ∞, where g² is the type IIA string coupling. We find that the BPS mass formulas agree, and reproduce the BPS mass spectrum of the (q₁, q₂) string bound state. In the non-BPS sector, the respective mass formulas apply in different corners of the moduli space. The axiomatic requirement of T-duality in M-theory permits to derive a discrete mass spectrum in a (thin torus) region where standard supermembrane theory undergoes instabilities.