We present UES @ WHT high-resolution spectra of the low-mass X-ray binary ( LMXB ) Cygnus X-2 .
We have derived the stellar parameters of the secondary star using \chi ^ { 2 } minimisation procedure , and taking into account any possible veiling from the accretion disk .
We determine a metallicity higher than solar ( [ Fe/H ] = 0.27 \pm 0.19 ) , as seen also in the neutron star X-ray binary Centaurus X-4 .
The high-quality of the secondary ’ s spectrum allow us to determine the chemical abundances of O , Mg , Si , Ca , S , Ti , Fe and Ni .
We found that some \alpha -elements ( Mg , Si , S , Ti ) are enhanced , consistent with a scenario of contamination of the secondary star during the supernova event .
Surprisingly oxygen appears to be under-abundant , whereas enhanced abundances of Fe and Ni are measured .
Assuming that these abundances come from matter that has been processed in the SN and then captured by the secondary star , we explore different SN explosion scenarios with diverse geometries .
A non-spherically symmetric SN explosion , with a low mass cut , seems to reproduce better the observed abundance pattern of the secondary star compared to the spherical case .