Small $E_8$ Instantons and Tensionless Non-critical Strings

Abstract

We use T-duality to extract information on an instanton of zero size in the $E_8\times E_8$ heterotic string. We discuss the possibility of the appearance of a tensionless self-dual non-critical string through an implementation of the mechanism suggested by Strominger of two coincident 5-branes. It is argued that when an instanton shrinks to zero size a tensionless noncritical string appears at the core of the instanton.