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The luminous low-mass X-ray binary X2127+119 in the core of the globularcluster M15 (NGC 7078), which has an orbital period of 17 hours, has long beenassumed to contain a donor star evolving off the main sequence, with a mass of0.8 solar masses (the main-sequence turn-off mass for M15). We presentorbital-phase-resolved spectroscopy of X2127+119 in the H-alpha and He I 6678spectral region, obtained with the Hubble Space Telescope. We show that thesedata are incompatible with the assumed masses of X2127+119's component stars.The continuum eclipse is too shallow, indicating that much of the accretiondisc remains visible during eclipse, and therefore that the size of the donorstar relative to the disc is much smaller in this high-inclination system thanthe assumed mass-ratio allows. Furthermore, the flux of X2127+119's He I 6678emission, which has a velocity that implies an association with the stream-discimpact region, remains unchanged through eclipse, implying that material fromthe impact region is always visible. This should not be possible if thepreviously-assumed mass ratio is correct. In addition, we do not detect anyspectral features from the donor star, which is unexpected for a 0.8 solar-masssub-giant in a system with a 17-hour period.Comment: 6 pages, 4 figures, accepted by A&

A new mass-ratio for the X-ray binary X2127+119 in M 15?