Context : Cool stars , companions to compact objects , are known to show Li abundances which are high compared to field stars of the same spectral type , which are heavily Li depleted .
This may be due either to Li production or Li preservation in theses systems .

Aims : To measure the lithium isotopic ratio in the companion star of the neutron star X-ray binary Cen X-4 .

Methods : We use UVES spectra obtained in years 2000 and 2004 around the orbital quadratures .
The spectra are analysed with spectrum synthesis techniques and the errors estimated with Monte Carlo simulations .

Results : We measure A ( Li ) = 2.87 \pm 0.20 and ^ { 6 } Li/ ^ { 7 } Li = 0.12 ^ { +0.08 } _ { -0.05 } at 68 % confidence level .
We also present updated system parameters with a refined determination of the orbital period and component masses i.e . 1.14 \pm 0.45 \hbox { M } _ { \odot } and 0.23 \pm 0.10 \hbox { M } _ { \odot } for the neutron star and companion , respectively .

Conclusions : In our view the low level of ^ { 6 } Li favours Li preservation scenarios , although Li production mechanisms can not be ruled out .
In the case of preservation , no Li is freshly created in the binary , but the tidally-locked companion has preserved its original Li by some mechanism , possibly inhibited destruction due to its fast rotation .