  We study a generally covariant model in which local Lorentz invariance is
broken "spontaneously" by a dynamical unit timelike vector field $u^a$---the
"aether". Such a model makes it possible to study the gravitational and
cosmological consequences of preferred frame effects, such as ``variable speed
of light" or high frequency dispersion, while preserving a generally covariant
metric theory of gravity. In this paper we restrict attention to an action for
an effective theory of the aether which involves only the antisymmetrized
derivative $\nabla_{[a}u_{b]}$. Without matter this theory is equivalent to a
sector of the Einstein-Maxwell-charged dust system. The aether has two massless
transverse excitations, and the solutions of the model include all vacuum
solutions of general relativity (as well as other solutions). However, the
aether generally develops gradient singularities which signal a breakdown of
this effective theory. Including the symmetrized derivative in the action for
the aether field may cure this problem.
