  We report the discovery of variable circularly polarised radio emission
associated with relativistic ejections from GRS 1915+105, based on observations
with the Australia Telescope Compact Array (ATCA) and the Multi-Element
Radio-Linked Interferometer Network (MERLIN). Following a radio flare in 2001
January, significant and variable circular polarisation, at a fractional level
of 0.2-0.4%, was measured with ATCA at four frequencies between 1-9 GHz.
Following an additional outburst 65 days later in 2001 March, further ATCA
observations measured a comparable sign and level of circular polarisation at
two frequencies. At this second epoch, contemporaneous MERLIN observations
directly imaged a relativistic ejection event and allowed us to confidently
associate both the circularly and linearly polarised emission with the
relativistic ejecta, allowing a detailed measurement of the full polarisation
properties in the optically thin phase. The fractional circular polarisation
spectrum appears to flatten at higher frequencies in 2001 January, when there
is strong evidence for multiple components at different optical depths. While
we cannot conclusively distinguish between synchrotron or propagation-induced
conversion as the origin of the circularly polarised component, we do not
consider that coherent or birefringent scintillation mechanisms are likely. The
implication is therefore that the ejections from GRS 1915+105 are associated
with a significant population of low-energy electrons, with associated
consequences for the energetics of relativistic ejection events. [abridged]
