  We report measurements of the temperature dependence of the critical current
in Josephson junctions consisting of conventional superconducting banks of Nb
and a weakly ferromagnetic interlayer of a Cu$_x$Ni$_{1-x}$ alloy, with $x$
around 0.5. With decreasing temperature $I_c$ generally increases, but for
specific thicknesses of the ferromagnetic interlayer, a maximum is found
followed by a strong decrease down to zero, after which $I_c$ rises again. Such
a sharp cusp can only be explained by assuming that the junction changes from a
0-phase state at high temperatures to a $\pi$-phase state at low temperatures.
