Temperature instabilities in N=4 strings

An effective supergravity description of all instabilities of N-4 = 4 super strings is derived. The construction is based on the N-4 = 4 BPS mass formu la at finite temperature and uses the properties of N-4 = 4 gauged supergra vity. It provides the boundaries of the various thermal phases in the non-p erturbative moduli space. It also draws a precise picture of the dynamics i n the high-temperature heterotic phase. This brief contribution summarizes results obtained in collaboration with I. Antoniadis and C. Kounnas.