Point-like instantons and the spin(32)/Z(2) heterotic string

We consider heterotic string theories compactified on a K3 surface which lead to an unbroken perturbative gauge group of Spin(32)/Z(2). All solutions obtained are combinations of two types of point-like instanton - one ''simple type'' as discovered by Witten and a new type associated to the ''generalized second Stiefel-Whitney class'' as introduced by Berkooz et al, The new type of instanton is associated to an enhancement of the gauge symmetry by Sp(4) and the addition of a massless tensor supermultiplet. It is shown that if four simple instantons coalesce at an orbifold point in the K3 surface then a massless tensor field appears which may be used to interpolate between the two types of instanton. By allowing various combinations of point-like instantons to coalesce, large gauge groups (e.g., rank 128) with many massless tensor supermultiplets result. The analysis is done in terms of F-theory. (C) 1997 Elsevier Science B.V.