Spectroscopy of the M15 X-ray source: discovery of binary motion and an unusual systemic velocity

We present the first phase-resolved optical spectroscopy of AC211, the optical counterpart of the X-ray source in the core of M15. The spectrum is dominated by the variable and complex He IIλ4686 emission, but we have also found He I in absorption. By following the motion of He Iλ4471 we have discovered the binary period to be 9.1±0.5 hr, with a semi-amplitude of 40±15 km s⁻¹. If this is interpreted as reflecting the orbital motion of the compact object, either the primary is $$10-14 M_\odot$$, or the secondary is a very light ($$\lt0.4 M_\odot$$) giant. But, most remarkably, we find that the mean He I velocity is blueshifted by ∼150±10km s⁻¹ with respect to the rest of the cluster.