Helium abundances and its radial gradient from the spectra of HII regions and ring nebulae of the Milky Way.

We determine the radial abundance gradient of helium in the disc of the Galaxy from published spectra of 19 $\text{H}\thinspace \text{II}$ regions and ring nebulae surrounding massive O stars. We revise the Galactocentric distances of the objects considering {\it Gaia} DR2 parallaxes and determine the physical conditions and the ionic abundance of He$^{+}$ in a homogeneous way, using between 3 and 10 $\text{He}\thinspace \text{I}$ recombination lines in each object. We estimate the total He abundance of the nebulae and its radial abundance gradient using four different ICF(He) schemes. The slope of the gradient is always negative and weakly dependent on the ICF(He) scheme, especially when only the objects with log($\eta$) $<$ 0.9 are considered. The slope values go from $-$0.0078 to $-$0.0044 dex kpc$^{-1}$, consistent with the predictions of chemical evolution models of the Milky Way and chemodynamical simulations of disc galaxies. Finally, we estimate the abundance deviations of He, O and N in a sample of ring nebulae around Galactic WR stars, finding a quite similar He overabundance of about +0.24 $\pm$ 0.11 dex in three stellar ejecta ring nebulae.