By T-dualizing space-filling D-branes in IIB orientifold compactifications along the three non-internal spatial directions, we obtain black hole bound states living in a universe with a gauged spatial reflection symmetry. We call these objects orient
iholes. 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.
Three dimensional topologically massive gravity (TMG) with a negative cosmological constant -ell^{-2} and positive Newton constant G admits an AdS_3 vacuum solution for any value of the graviton mass mu. These are all known to be perturbatively unsta
ble except at the recently explored chiral point muell=1. However we show herein that for every value of muell< 3 there are two other (potentially stable) vacuum solutions given by SL(2,R)x U(1)-invariant warped AdS_3 geometries, with a timelike or spacelike U(1) isometry. Critical behavior occurs at muell=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 muell>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 muell>3, the warped AdS_3 ground state of TMG is holographically dual to a 2D boundary CFT with central charges c_R={15(muell)^2+81over Gmu((muell)^2+27)} and c_L={12 muell^2over G((muell)^2+27)}.
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.
