  We investigate the atmospheric composition of the long period ($P_{\rm orb}=$
10 days), eccentric exo-Saturn WASP-117b. WASP-117b could be in atmospheric
temperature and chemistry similar to WASP-107b. In mass and radius WASP-117b is
similar to WASP-39b, which allows a comparative study of these planets.
  We analyze a near-infrared transmission spectrum of WASP-117b taken with
Hubble Space Telescope/WFC3 G141, which was reduced with two independent
pipelines. High resolution measurements were taken with VLT/ESPRESSO in the
optical. We report the robust ($3\sigma$) detection of a water spectral
feature. Using a 1D atmosphere model with isothermal temperature, uniform cloud
deck and equilibrium chemistry, the Bayesian evidence of a retrieval analysis
of the transmission spectrum indicates a preference for a high atmospheric
metallicity ${\rm [Fe/H]}=2.58^{+0.26}_{-0.37}$ and clear skies. The data are
also consistent with a lower-metallicity composition ${\rm [Fe/H]}<1.75$ and a
cloud deck between $10^{-2.2} - 10^{-5.1}$ bar, but with weaker Bayesian
preference. We retrieve a low CH$_4$ abundance of $<10^{-4}$ volume fraction
within $1 \sigma$ and $<2\cdot 10^{-1}$ volume fraction within $3 \sigma$. We
cannot constrain the equilibrium temperature between theoretically imposed
limits of 700 and 1000~K. Further observations are needed to confirm quenching
of CH$_4$ with $K_{zz}\geq 10^8$~cm$^2$/s. We report indications of Na and K in
the VLT/ESPRESSO high resolution spectrum with substantial Bayesian evidence in
combination with HST data.

Earth and Planetary Astrophysics;Solar and Stellar Astrophysics