The relatively small family of ultra-compact X-ray binary systems is of great interest for many areas of astrophysics .
We report on a detailed X-ray spectral study of the persistent neutron star low mass X-ray binary 1RXS J170854.4-321857 .
We analysed two XMM-Newton observations obtained in late 2004 and early 2005 when , in agreement with previous studies , the system displayed an X-ray luminosity ( 0.5–10 keV ) of \sim 1 \times 10 ^ { 36 } \mathrm { erg~ { } s } ^ { -1 } .
The spectrum can be described by a Comptonized emission component with \Gamma \sim 1.9 and a distribution of seed photons with a temperature of \sim 0.23 keV .
A prominent residual feature is present at soft energies , which is reproduced by the absorption model if over-abundances of Ne and Fe are allowed .
We discuss how similar observables , that might be attributed to the peculiar ( non-solar ) composition of the plasma donated by the companion star , are a common feature in confirmed and candidate ultra-compact systems .
Although this interpretation is still under debate , we conclude that the detection of these features along with the persistent nature of the source at such low luminosity and the intermediate-long burst that it displayed in the past confirms 1RXS J170854.4-321857as a solid ultra-compact X-ray binary candidate .