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

The luminous low-mass X-ray binary X2127+119 in the core of the globular cluster M 15 (NGC 7078), which has an orbital period of 17 hours, has long been assumed to contain a donor star evolving off the main sequence, with a mass of 0.8 M⊙ (the main-sequence turn-off mass for M 15). We present orbital-phase-resolved spectroscopy of X2127+119 in the Ha and He I λ6678 spectral region, obtained with the Hubble Space Telescope. We show that these data are incompatible with the assumed masses of X2127+119's component stars. The continuum eclipse is too shallow, indicating that much of the accretion disc remains visible during eclipse, and therefore that the size of the donor star relative to the disc is much smaller in this high-inclination system than the assumed mass-ratio allows. Furthermore, the flux of X2127+119's He I λ6678 emission, which has a velocity that implies an association with the stream-disc impact region, remains unchanged through eclipse, implying that material from the impact region is always visible. This should not be possible if the previously-assumed mass ratio is correct. In addition, we do not detect any spectral features from the donor star, which is unexpected for a 0.8-M⊙ sub-giant in a system with a 17-h period.