We give a local characterization of codimension two submanifolds which are
marginally trapped in Robertson-Walker spaces, in terms of an algebraic
equation to be satisfied by the height function. We prove the existence of a
large number of local solutions. We refine the description in the case of
curves with null acceleration in three-dimensional spaces Robertson-Walker
spaces, and in the case of codimension two submanifolds whose second
fundamental form is nullComment: 16 pages, Bibliography updated and small correction
