2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/4807In the hard X-ray spectra of some X-ray binaries line features around 500 keV are detected. We interpret these as arising from pair annihilation in relativistic outflows leading to a significant Doppler shift of the frequencies of the lines. We show that a small fraction of pairs escaping the annihilation region may give rise to the radio synchrotron emission observed in Nova Muscae 1991.4 pages, 1 figure. To appear in the "Proceedings of the 4th Microquasar Workshop", eds. Ph Durouchoux, Y. Fuchs and J. Rodriguez, published by the Center for Space Physics: KolkataAstrophysicstext