We 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 (2009) we find a good agreement with results reported in Casares et al. (1998). We measure $P=9.84450\pm0.00019$ day, $K_2=86.5\pm1.2$ km s$^{ -1}$ and $V \sin i=33.7\pm0.9$ km s$^{ -1}$. These values imply $q=M_{ 2}/M_{ 1}=0.34 \pm 0.02$ and $M_{ 1}=1.71\pm 0.21$ M$_{ \odot}$ (for $i=62.5 \pm 4^{ \circ}$). 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 $K_2$ values.
Categories: astro-ph.GA