  The technique of transmission spectroscopy allows us to constrain the
chemical composition of the atmospheres of transiting exoplanets. It relies on
very high signal-to-noise spectroscopic (or spectrophotometric) observations
and is thus most suited for bright exoplanet host stars. In the era of TESS,
NGST and PLATO, more and more suitable targets, even for mid-sized telescopes,
are discovered. Furthermore, a wealth of archival data is available that could
become a basis for long-term monitoring of exo-atmospheres. We analyzed
archival HARPS spectroscopic time series of four host stars to transiting
bloated gas exoplanets, namely WASP-76b, WASP-127b, WASP-166b and KELT-11b,
searching for traces of sodium (sodium doublet), hydrogen (H$\alpha$,
H$\beta$), and lithium (670.8 nm). The archival data sets include spectroscopic
time series taken during transits. Comparing in- and out-of-transit spectra we
can filter out the stellar lines and investigate the absorption from the
planet. Simultaneously, the stellar activity is monitored using the Mg I and Ca
I lines. We independently detect sodium in the atmosphere of WASP-76b at a 7-9
$\sigma$ level. Furthermore, we report also at 4-8 $\sigma$ level of
significance the detection of sodium in the atmosphere of WASP-127b, confirming
earlier result based on low-resolution spectroscopy. The data show no sodium
nor any other atom at high confidence levels for WASP-166b nor KELT-11b,
hinting at the presence of thick high clouds.
