  We present the first measurement of charge-dependent directed flow in Cu+Au
collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV. The results are presented as a
function of the particle transverse momentum and pseudorapidity for different
centralities. A finite difference between the directed flow of positive and
negative charged particles is observed that qualitatively agrees with the
expectations from the effects of the initial strong electric field between two
colliding ions with different nuclear charges. The measured difference in
directed flow is much smaller than that obtained from the
parton-hadron-string-dynamics (PHSD) model, which suggests that most of the
electric charges, i.e. quarks and antiquarks, have not yet been created during
the lifetime of the strong electric field, which is of the order of, or less
than, 1fm/$c$.
