NEW PHASES OF STRING THEORY AND 6D RG FIXED-POINTS VIA BRANES AT ORBIFOLD SINGULARITIES

We discuss type IT and type I branes at general ADE type orbifold sing ularities. We show that there are new phases of type I or heterotic st ring theory in six dimensions, involving extra tensor multiplets, whic h arise when small instantons sit on orbifold singularities. The theor ies with extra tensor multiplets are explicitly constructed via orient ifolds. The world-volume theories in type IIB or type I five-branes at orbifold singularities lead to the existence of several infinite clas ses of six-dimensional, interacting, renormalization group fixed point theories. (C) 1997 Elsevier Science B.V.