On the mass of the neutron star in Cyg X-2

International audienceWe present new high-resolution spectroscopy of the low-mass X-ray binary Cyg X-2, which enables us to refine the orbital solution and rotational broadening of the donor star. In contrast with Elebert et al., we find a good agreement with results reported in Casares et al. We measure P = 9.84450 +/- 0.00019 d, K2 = 86.5 +/- 1.2 kms-1 and V sini = 33.7 +/- 0.9 kms-1. These values imply q = M2/M1 = 0.34 +/- 0.02 and M1 = 1.71 +/- 0.21Msolar (for ). Therefore, the neutron star in Cyg X-2 can be more massive than canonical. We also find no evidence for irradiation effects in our radial velocity curve which could explain the discrepancy between Elebert et al.'s and our K2 values