  From an operad C with an action of a group G, we construct new operads using
the homotopy fixed point and orbit spectra. These new operads are shown to be
equivalent when the generalized G-Tate cohomology of C is trivial. Applying
this theory to the little disk operad C_2 (which is an S^1 operad) we obtain
variations on Getzler's gravity operad, which we show governs the Chas-Sullivan
string bracket.
