We present phase resolved optical spectroscopy and Doppler tomography of V1341 Cygni , the optical counterpart to the neutron star low mass X-ray binary Cygnus X-2 .
We derive a radial velocity curve for the secondary star , finding a projected radial velocity semi-amplitude of K _ { 2 } = 79 \pm 3 km s ^ { -1 } , leading to a mass function of 0.51 \pm 0.06 M _ { \sun } , \sim 30 % lower than the previous estimate .
We tentatively attribute the lower value of K _ { 2 } ( compared to that obtained by other authors ) to variations in the X-ray irradiation of the secondary star at different epochs of observations .
The limited phase coverage and/or longer timebase of previous observations may also contribute to the difference in K _ { 2 } .
Our value for the mass function implies a primary mass of 1.5 \pm 0.3 M _ { \sun } , somewhat lower than previous dynamical estimates , but consistent with the value found by analysis of type-I X-ray bursts from this system .
Our Doppler tomography of the broad He ii \lambda 4686 line reveals that most of the emission from this line is produced on the irradiated face of the donor star , with little emission from the accretion disc .
In contrast , the Doppler tomogram of the N iii \lambda 4640.64 Bowen blend line shows bright emission from near the gas stream/accretion disc impact region , with fainter emission from the gas stream and secondary star .
This is the first LMXB for which the Bowen blend is dominated by emission from the gas stream/accretion disc impact region , without comparable emission from the secondary star .
This has implications for the interpretation of Bowen blend Doppler tomograms of other LMXBs for which the ephemeris may not be accurately known .