Modulation of the waterfall by a gauge field
  We present the first complete calculation of the curvature perturbation
generated during the hybrid inflation waterfall, caused by the coupling of the
waterfall field to a gauge field A whose kinetic function f^2 depends on the
inflaton field. We impose an upper bound on the field W=f*A which ensures that
it has a negligible effect before the waterfall. We confirm the claim of Soda
and Yokoyama, that the perturbation \delta W generates a statistically
anisotropic spectrum and bispectrum, which could easily be observable. We also
discover a new phenomenon, whereby the time-dependent `varyon' field W causes
the inflaton contribution to vary during the waterfall. The varyon mechanism
might be implemented also with a scalar field and might not involve the
waterfall.
