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.