Brγ line observables. Top: normalised spectral line profile averaged over the four telescopes with GRAVITY. Bottom: differential visibilities from the UT1-UT4 baseline of the CI Tau observation in 2021. The small blue dots and error bars represent the total visibility. The larger coloured dots indicate the pure line visibilities after the subtraction of the continuum contribution (see Eq. (2)). The continuum estimate and the associated uncertainty are shown as red dashed lines. The Gaussian model used to fit the total visibility is represented as a blue line.