  In the hard X-ray spectra of some X-ray binaries line features at around 500
keV are detected. We interpret these as arising from pair annihilation in
relativistic outflows leading to a significant Doppler shift of the lines'
frequencies. We show how this can be used to accurately determine the bulk
velocity and orientation to the line of sight of the outflows. Constraints on
the energy requirements of such outflows are also derived. Furthermore, we show
that a small fraction of pairs escaping the annihilation region may give rise
to the radio synchrotron emission observed in some of these objects. We apply
these ideas to the hard X-ray and radio observations of Nova Muscae 1991. In
this object, the energy requirements seem to rule out a large proton fraction
in the outflows.
