  We explore the role of the Cremmer-Scherk mechanism in the context of low
energy effective string theory by coupling the antisymmetric 3-form gauge
potential to an Abelian gauge potential carrying weak hypercharge. The theory
admits a class of exact self-gravitating solutions in the spontaneously broken
phase in which the dual fields acquire massive perturbative modes. Despite the
massive nature of these fields they admit non-perturbative progressive
longitudinal modes that together with pp-type gravitational waves travel in a
direction of a line source at the speed of light.
