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

Understand

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$^{-1}$.