  Charged massive scalar fields are considered in the gravitational and
electromagnetic field produced by a dyonic black hole with a cosmic string
along its axis of symmetry. Exact solutions of both angular and radial parts of
the covariant Klein-Gordon equation in this background are obtained, and are
given in terms of the confluent Heun functions. The role of the presence of the
cosmic string in these solutions is showed up. From the radial solution, we
obtain the exact wave solutions near the exterior horizon of the black hole,
and discuss the Hawking radiation spectrum and the energy flux.
