BPS solutions and new phases of finite-temperature strings

All high-temperature phases of the known N = 4 superstrings in five dimensi ons can be described by the universal thermal potential of an effective fou r-dimensional supergravity. This theory, in addition to three moduli s, t, u, contains non-trivial winding modes that become massless in certain regio ns of the thermal moduli space, triggering the instabilities at the Hagedor n temperature. In this context, we look for exact domain wall solutions of first order BPS equations. These solutions preserve half of the supersymmet ries, in contrast to the usual finite-temperature weak-coupling approximati on, and as such may constitute a new phase of finite-temperature superstrin gs. We present exact solutions for the type IIA and type IIB theories and f or a self-dual hybrid type II theory. While for the heterotic case the gene ral solution cannot be given in closed form, we still present a complete pi cture and a detailed analysis of the behaviour around the weak and strong c oupling limits and around certain critical points. In all cases these BPS s olutions have no instabilities at any temperature. Finally, we address the physical meaning of the resulting geometries within the contexts of supergr avity and string theory. (C) 2001 Elsevier Science B.V. All rights reserved .