  We present near-infrared linear spectropolarimetry of a sample of persistent
X-ray binaries, Sco X-1, Cyg X-2 and GRS1915+105. The slopes of the spectra are
shallower than what is expected from a standard steady-state accretion disc,
and can be explained if the near-infrared flux contains a contribution from an
optically thin jet. For the neutron star systems, Sco X-1 and Cyg X-2, the
polarization levels at 2.4um are 1.3+/-0.10% and 5.4+/-0.7% respectively which
is greater than the polarization level at 1.65um. This cannot be explained by
interstellar polarization or electron scattering in the anisotropic environment
of the accretion flow. We propose that the most likely explanation is that this
is the polarimetric signature of synchrotron emission arising from close to the
base of the jets in these systems. In the black hole system GRS1915+105 the
observed polarization, although high (5.0+/-1.2% at 2.4um), may be consistent
with interstellar polarization. For Sco X-1 the position angle of the radio jet
on the sky is approximately perpendicular to the near-infrared position angle
(electric vector), suggesting that the magnetic field is aligned with the jet.
These observations may be a first step towards probing the ordering, alignment
and variability of the outflow magnetic field in a region closer to the central
accreting object than is observed in the radio band.
