TYPE-II DUALITY SYMMETRIES IN 6 DIMENSIONS

We discuss the different discrete duality symmetries in six dimensions that act within and between (i) the 10-dimensional heterotic string c ompactified on T-4, (ii) the 10-dimensional type IIA string compactifi ed on K3 and (iii) the 10-dimensional type IIB string compactified on K3, In particular we show that the underlying group-theoretical struct ure of these discrete duality symmetries is determined by the proper c ubic group C/Z(2). Our group theoretical interpretation leads to simpl e rules for constructing the explicit form of the different discrete t ype II duality symmetries in an arbitrary background. The explicit dua lity rules we obtain are applied to construct dual versions of the 6-d imensional chiral null model.