  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]$=0.27\pm0.19$), 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.
