Open heterotic strings

We classify potential cosmic strings according to the topological chargemeasurable outside the string core. We conjecture that in string theory it isthis charge that governs the stability of long strings. This would imply thatthe SO(32) heterotic string can have endpoints, but not the E_8 x E_8 heteroticstring. We give various arguments in support of this conclusion.Comment: 15 pages. v.2: typos, references correcte