[ Star ]

Cygnus X-2 is one of the brightest and longest known X-ray sources. We present high-resolution optical spectroscopy of Cyg X-2 obtained over 4 yr, which gives an improved mass function of 0.69 (cid:53) 0.03 M (cid:44) (1 (cid:106) ). In addition, we resolve the rotationally broadened absorption features of the secondary star for the first time, deriving a rotation speed of km s (cid:50) 1 (1 (cid:106) ), which leads to a mass ratio of (cid:118) sin i (cid:53) 34.2 (cid:53) 2.5 q (cid:53) M / M (cid:53) c X (cid:53) 0.04 (1 (cid:106) , assuming a tidally locked and Roche lobe-filling secondary). Hence, with the lack of X-ray 0.34eclipses (i.e., ) we can set firm 95% confidence lower limits to the neutron star mass of M (cid:44) (cid:67) i (cid:38) 73 M (cid:49) 1.27 X and to the companion star mass of M (cid:44) . However, by additionally requiring that the companion must M (cid:49) 0.39 c exceed 0.75 M (cid:44) (as required theoretically to produce a steady low-mass X-ray binary), then M (cid:44) and M (cid:49) 1.88 X (95% confidence lower and upper limit, respectively), thereby making Cyg X-2 the highest mass neutron (cid:67) i (cid:33) 61 star measured to date. If confirmed, this would set significant constraints on the equation of state of nuclear matter.