Investigating the BPS spectrum of non-critical En strings

Abstract We use the effective action of the En non-critical strings to study its BPS spectrum for 0 ⩽ n ⩽ 8. We show how to introduce mass parameters, or Wilson lines, into the effective action, and then perform the appropriate asymptotic expansions that yield the BPS spectrum. The result is the En character expansion of the spectrum, and is equivalent to performing the mirror map on a Calabi-Yau with up to nine Kahler moduli. This enables a much more detailed examination of the En structure of the theory, and provides extensive checks on the effective action description of the non-critical string. We extract some universal (En-independent) information concerning the degeneracies of BPS excitations.