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 E8 x E8 and the Spin(32)/Z2 heterotic string. As a simple application of the method we derive some basic properties of the non-perturbative physics of collections of E8 or Spin(32)/Z2 point-like instantons sitting at ADE singularities on a K3 surface. (orig.)