  Eta Car is one of the most intriguing luminous blue variables in the Galaxy.
Observations and models at different wavelengths suggest a central binary with
a 5.54 yr period residing in its core. 2D and 3D radiative transfer and
hydrodynamic simulations predict a primary with a dense and slow stellar wind
that interacts with the faster and lower density wind of the secondary. The
wind-wind collision scenario suggests that the secondary's wind penetrates the
primary's wind creating a low-density cavity in it, with dense walls where the
two winds interact. We aim to trace the inner ~5-50 au structure of Eta Car's
wind-wind interaction, as seen through BrG and, for the first time, through the
He I 2s-2p line. We have used spectro-interferometric observations with GRAVITY
at the VLTI. Our modeling of the continuum allows us to estimate its FWHM
angular size close to 2 mas and an elongation ratio of 1.06 +/- 0.05 over a PA
= 130 +/- 20 deg. Our CMFGEN modeling helped us to confirm that the role of the
secondary should be taken into account to properly reproduce the observed BrG
and He I lines. Chromatic images across BrG reveal a southeast arc-like
feature, possibly associated to the hot post-shocked winds flowing along the
cavity wall. The images of He I 2s-2p served to constrain the 20 mas structure
of the line-emitting region. The observed morphology of He I suggests that the
secondary is responsible for the ionized material that produces the line
profile. Both the BrG and the He I 2s-2p maps are consistent with previous
hydrodynamical models of the colliding wind scenario. Future dedicated
simulations together with an extensive interferometric campaign are necessary
to refine our constraints on the wind and stellar parameters of the binary,
which finally will help us predict the evolutionary path of Eta Car.
