Point-like instantons on K3 orbifolds

The map between the moduli space of F-theory (or type II string) compactifications and heterotic string compactifications can be considerably simplified by using ''stable degenerations''. We discuss how this method applies to both the E-8 x E-8 and the Spin(32)/Z(2) heterotic string. As a simple application of the method we derive some basic properties of the non-perturbative physics of collections of E-8 or Spin(32)/Z(2) point-like instantons sitting at ADE singularities on a K3 surface. (C) 1997 Elsevier Science B.V. PACS: 02.10.Rn; 04.50.+h; 11.10.Kk; 11.25.Sq.