Some interesting violations of the Breitenlohner-Freedman bound

We demonstrate that AdS_5 x T^{pq} is unstable, in the sense of Breitenlohner and Freedman, for unequal p and q. This settles, negatively, the long-standing question of whether the T^{pq} manifolds for unequal p and q might correspond to non-supersymmetric fixed points of the renormalization group. We also show that the AdS_3 x S^7 vacuum of Sugimotos USp(32) open string theory is unstable. This explains, at a heuristic level, the apparent absence of a heterotic string dual.