  Given a C*-dynamical system (A,G,\alpha), we discuss conditions under which
subalgebras of the multiplier algebra M(A) consisting of fixed points for
\alpha are Morita-Rieffel equivalent to ideals in the crossed product of A by
G. In case G is abelian we also develop a spectral theory, giving a necessary
and sufficient condition for \alpha to be equivalent to the dual action on the
cross-sectional C*-algebra of a Fell bundle. In our main application we show
that a proper action of an abelian group on a locally compact space is
equivalent to a dual action.
