Time-resolved spectroscopy of the M15 X-ray binary AC211/X2127+119

We present time-resolved spectroscopy acquired during two epochs (spaced apart by ∼ 15 d) of the eclipsing low-mass X-ray binary AC211/X2127+119 in the globular cluster M15. The spectra show variations in the He II λ4686 emission line not only modulated on the orbital period, but also on time-scales of a few days. During the first epoch of observation, the emission line shows a strong S-wave superimposed on the average double-peaked profile. The line exhibits no evidence of rotational disturbance at the orbital phases when the eclipse is observed in the optical continuum. During the second epoch, no double-peak or S-wave component is present. The He I absorption lines detected by other authors are not present in our spectra. A Doppler image of He II λ4686 for the first epoch supports the presence of the accretion disc. No hotspot is detected, although enhanced emission at V_x = 30 km s_-1, V_Y = 160 km s_-1 is observed. We discuss the implications of this emission in the context of an X-ray heated donor star, in which case a high mass ratio and neutron star primary are implied. Finally, we speculate on the possibility of a misaligned secondary star in AC211.