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Exoplanets orbiting very close to their host star are strongly irradiated.This can lead the upper atmospheric layers to expand and evaporate into space.The metastable helium (HeI) triplet at 1083.3nm has recently been shown to be apowerful diagnostic to probe extended and escaping exoplanetary atmosphere. Weperform high-resolution transmission spectroscopy of the transiting hot JupiterHD189733b with the GIARPS (GIANO-B + HARPS-N) observing mode of the TelescopioNazionale Galileo, taking advantage of the simultaneous optical+near infraredspectral coverage to detect HeI in the planet's extended atmosphere and togauge the impact of stellar magnetic activity on the planetary absorptionsignal. Observations were performed during five transit events of HD189733b. Bycomparison of the in- and out-of-transit GIANO-B observations we computehigh-resolution transmission spectra, on which we perform equivalent widthmeasurements and light-curves analyses to gauge the excess in-transitabsorption in the HeI triplet. We detect an absorption signal during all fivetransits. The mean in-transit absorption depth amounts to 0.75+/-0.03%. Wedetect night-to-night variations in the HeI absorption signal likely due to thetransit events occurring in presence of stellar surface inhomogeneities. Weevaluate the impact of stellar-activity pseudo-signals on the true planetaryabsorption using a comparative analysis of the HeI and the H$\alpha$ lines. Weinterpret the time-series of the HeI absorption lines in the three nights notaffected by stellar contamination -exhibiting a mean in-transit absorptiondepth of 0.77+/-0.04%- using a 3-d atmospheric code. Our simulations suggestthat the helium layers only fill part of the Roche lobe. Observations can beexplained with a thermosphere heated to $\sim$12000 K, expanding up to$\sim$1.2 planetary radii, and losing $\sim$1 g/s of metastable helium.

The GAPS programme at TNG XXII. The GIARPS view of the extended helium atmosphere of HD189733 b accounting for stellar activity