  The space of monic centered cubic polynomials with marked critical points is
isomorphic to C^2. For each n>0, the locus Sn formed by all polynomials with a
specified critical point periodic of exact period n forms an affine algebraic
set. We prove that Sn is irreducible, thus giving an affirmative answer to a
question posed by Milnor. (This manuscript has been withdrawn)
