  We investigate the current-phase relation of S/F/S junctions near the
crossover between the 0 and the pi ground states. We use Nb/CuNi/Nb junctions
where this crossover is driven both by thickness and temperature. For a certain
thickness a non-zero minimum of critical current is observed at the crossover
temperature. We analyze this residual supercurrent by applying a high frequency
excitation and observe the formation of half-integer Shapiro steps. We
attribute these fractional steps to a doubling of the Josephson frequency due
to a sin(2*phi) current-phase relation. This phase dependence is explained by
the splitting of the energy levels in the ferromagnetic exchange field.
