Three dimensional topologically massive gravity (TMG) with a negative cosmological constant −ℓ −2 and positive Newton constant G admits an AdS 3 vacuum solution for any value of the graviton mass µ. These are all known to be perturbatively unstable except at the recently explored chiral point µℓ = 1. However we show herein that for every value of µℓ = 3 there are two other (potentially stable) vacuum solutions given by SL(2, R) × U (1)-invariant warped AdS 3 geometries, with a timelike or spacelike U (1) isometry. Critical behavior occurs at µℓ = 3, where the warping transitions from a stretching to a squashing, and there are a pair of warped solutions with a null U (1) isometry. For µℓ > 3, there are known warped black hole solutions which are asymptotic to warped AdS 3 . We show that these black holes are discrete quotients of warped AdS 3 just as BTZ black holes are discrete quotients of ordinary AdS 3 . Moreover new solutions of this type, relevant to any theory with warped AdS 3 solutions, are exhibited. Finally we note that the black hole thermodynamics is consistent with the hypothesis that, for µℓ > 3, the warped AdS 3 ground state of TMG is holographically dual to a 2D boundary CFT with central charges c R = 15(µℓ) 2 +81 Gµ((µℓ) 2 +27) and c L = 12µℓ 2 G((µℓ) 2 +27) .

We investigate the hypothesis that the higher-derivative corrections always make extremal non-supersymmetric black holes lighter than the classical bound and self-repulsive. This hypothesis was recently formulated in the context of the socalled swampland program. One of our examples involves an extremal heterotic black hole in four dimensions. We also calculate the effect of general four-derivative terms in Maxwell-Einstein theories in D dimensions. The results are consistent with the conjecture.

We derive extremal black hole solutions for a variety of four dimensional models which, after Kaluza-Klein reduction, admit a description in terms of 3D gravity coupled to a sigma model with symmetric target space. The solutions are in correspondence with certain nilpotent generators of the isometry group. In particular, we provide the exact solution for a non-BPS black hole with generic charges and asymptotic moduli in N = 2 supergravity coupled to one vector multiplet. Multi-centered solutions can also be generated with this technique. It is shown that the non-supersymmetric solutions lack the intricate moduli space of bound configurations that are typical of the supersymmetric case.

By T-dualizing space-filling D-branes in 4d IIB orientifold compactifications along the three noninternal spatial directions, we obtain black hole bound states living in a universe with a gauged spatial reflection symmetry. We call these objects orientiholes. The gravitational entropy of various IIA orientihole configurations provides an "experimental" estimate of the number of vacua in various sectors of the IIB landscape. Furthermore, basic physical properties of orientiholes map to (sometimes subtle) microscopic features, thus providing a useful alternative viewpoint on a number of issues arising in D-brane model building. More generally, we give orientihole generalizations of recently derived wall crossing formulae, and conjecture a relation to the topological string analogous to the OSV conjecture, but with a linear rather than a quadratic identification of partition functions.1