  We present integral field spectroscopy of X2127+119, the luminous X-ray
binary in the globular cluster M15, obtained with INTEGRAL/WYFFOS on the
William Herschel Telescope. We find that tomograms of HeII 4686 line profiles
appear to be incompatible with the previously-assumed view of X2127+119, in
which the binary consists of a 1.4-solar-mass neutron star and a 0.8-solar-mass
sub-giant companion near the main-sequence turn-off for M15. Our data imply a
much smaller mass ratio M_2/M_x of 0.1. We find that models of X2127+119 with
black-hole compact objects give a poor fit to our data, while a neutron-star
compact object is consistent with the data, implying that X2127+119's companion
may have a much lower mass (~0.1 solar masses) than previously assumed. As an
0.1-solar-mass main-sequence star would be unable to fill its Roche lobe in a
binary with X2127+119's orbital period (17.1 hours), the companion is likely to
be the remnant of a significantly more massive star which has had most of its
envelope stripped away.
