  The non-central Cu + Au collisions can create strong out-of-plane magnetic
fields and in-plane electric fields. By using the HIJING model, we study the
general properties of the electromagnetic fields in Cu + Au collisions at 200
GeV and their impacts on the charge-dependent two-particle correlator
$\gamma_{q_1q_2}=<\cos(\phi_1+\phi_2-2\psi_{RP})>$ (see main text for
definition) which was used for the detection of the chiral magnetic effect
(CME). Compared with Au + Au collisions, we find that the in-plane electric
fields in Cu + Au collisions can strongly suppress the two-particle correlator
or even reverse its sign if the lifetime of the electric fields is long.
Combining with the expectation that if $\gamma_{q_1q_2}$ is induced by
elliptic-flow driven effects we would not see such strong suppression or
reversion, our results suggest to use Cu + Au collisions to test CME and
understand the mechanisms that underlie $\gamma_{q_1q_2}$.
