Reflection measurements on a one-dimensional single crystal of YBa 2 Cu 3 O 7−δ and transmission measurements on single crystals of Bi 2 Sr 2 CaCu 2 O 8 are reported which indicate optical circular effects below temperatures Ts somewhat above the respective superconducting temperatures Tc. The signs of rotation can be controlled by an external magnetic field. We interpret these results as strong evidence for a state with broken time reversal symmetry below Ts, as predicted by the theories of anyon or flux phase superconductivity.