Circularly polarised radio emission from GRS 1915+105 and other X-ray binaries

I report the detection of circular polarisation, associated with relativistic ejections, from the 'microquasar' GRS 1915+ 105. I further compare detections and limits of circular polarisation and circular-to-linear polarisation ratios in other X-ray binaries. Since in at least two cases the dominance of linear over circular polarisation or vice versa is a function of frequency, this seems to indicate that this is a strong function of depolarisation in the source. Furthermore, I note that circular polarisation has only been detected from sources whose jets lie close to the plane of the sky, whereas we have quite stringent limits on the circular polarisation of jets which lie close to the line of sight.