Context :

Aims : We investigate the atmospheric composition of the long period ( P _ { 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 .

Methods : We analyze a near-infrared transmission spectrum of WASP-117b taken with Hubble Space Telescope/WFC3 G141 , which was reduced with two independent pipelines to ensure a robust detection of the 3 \sigma -water spectrum .
High resolution measurements were taken with VLT/ESPRESSO in the optical .

Results : 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 { [ Fe / H ] } = 2.58 ^ { +0.26 } _ { -0.37 } and clear skies .
The data are also consistent with a lower-metallicity composition { [ 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 .
We can not constrain the 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 .

Conclusions :