  The study of exoplanet atmospheres is essential to understand the formation,
evolution and composition of exoplanets. The transmission spectroscopy
technique is playing a significant role in this domain. In particular, the
combination of state-of-the-art spectrographs at low- and high-spectral
resolution is key to our understanding of atmospheric structure and
composition. Two transits of the close-in sub Saturn-mass planet,WASP-127b,
have been observed with ESPRESSO in the frame of the Guaranteed Time
Observations Consortium. Transit observations allow us to study simultaneously
the system architecture and the exoplanet atmosphere. We found that this planet
is orbiting its slowly rotating host star (veq sin(i)=0.53+/-0.07 km/s) on a
retrograde misaligned orbit (lambda=-128.41+/-5.60 deg). We detected the sodium
line core at the 9-sigma confidence level with an excess absorption of
0.3+/-0.04%, a blueshift of 2.7+/-0.79 km/s and a FWHM of 15.18+/-1.75 km/s.
However, we did not detect the presence of other atomic species but set
upper-limits of only few scale heights. Finally, we put a 3-sigma upper limit,
to the average depth of the 1600 strongest water lines at equilibrium
temperature in the visible band, of 38 ppm. This constrains the cloud-deck
pressure between 0.3 and 0.5 mbar by combining our data with low-resolution
data in the near-infrared and models computed for this planet. To conclude,
WASP-127b, with an age of about 10 Gyr, is an unexpected exoplanet by its
orbital architecture but also by the small extension of its sodium atmosphere
(~7 scale heights). ESPRESSO allows us to take a step forward in the detection
of weak signals, thus bringing strong constraints on the presence of clouds in
exoplanet atmospheres. The framework proposed in this work can be applied to
search for molecular species and study cloud-decks in other exoplanets.
