Strings in a 2-d Extremal Black Hole

String theory on 2-d charged black holes corresponding to (SL(2)xU(1)_L)/U(1) exact asymmetric quotient CFTs are investigated. These backgrounds can be embedded, in particular, in a two dimensional heterotic string. In the extremal case, the quotient CFT description captures the near horizon physics, and is equivalent to strings in AdS_2 with a gauge field. Such string vacua possess an infinite space-time Virasoro symmetry, and hence enhancement of global space-time Lie symmetries to affine symmetries, in agreement with the conjectured AdS_2/CFT_1 correspondence. We argue that the entropy of these 2-d black holes in string theory is compatible with semi-classical results, and show that in perturbative computations part of an incoming flux is absorbed by the black hole. Moreover, on the way we find evidence that the 2-d heterotic string is closely related to the N=(2,1) string, and conjecture that they are dual.Comment: 1+22 pages, harvmac, 1 eps figure; v2: refs. added, typo correcte

On Non-Perturbative Results in Supersymmetric Gauge Theories - A Lecture

Some notions in non-perturbative dynamics of supersymmetric gauge theories are being reviewed. This is done by touring through a few examples.Comment: 34 pages, Late

Effective Actions and Gauge Field Stability

By studying an effective action description of the coupling of charged gauge fields in N=2 SU(n) supersymmetric Yang-Mills theories, we can describe regions of moduli space where one or more of these fields becomes unphysical. We discuss subtleties in the structure of the moduli space for SU(3).Comment: 13 pages, latex, no figure

Supersymmetric string vacua on AdS_3 x N

String backgrounds of the form AdS_3 x N that give rise to two dimensional spacetime superconformal symmetry are constructed.Comment: harvmac, 9 pages (minor changes for submission to journal