  We show that the simply-connected four-manifolds which admit locally linear,
homologically trivial actions by rank two finite abelian groups are
homeomorphic to connected sums of CP^2, -CP^2, and S^2 x S^2 (with one
exception: pseudofree Z_3 x Z_3 actions on the Chern manifold), and also
establish an equivariant decompostion theorem.
  This generalizes results from a 1970 paper by Orlik and Raymond, and
complements more recent work of Fintushel, Yoshida, and Huck on S^1 actions. In
each case, the simply-connected four-manifolds which support such actions are
essentially the same.
