Fitting of the visibility amplitude data.

Top: Visibility data from the UT3-UT2 baseline of the RU Lup observation in 2021. The coloured squares indicate the pure line visibilities after the extraction of the continuum contribution from the Brγ signal as laid out in Eq. A.1. Bottom: An ellipse representing the 2D emission region as projected onto the six baselines of GRAVITY in a single spectral channel. We used a Gaussian disk model to determine the projected half width at half maximum (HWHM) at each baseline and then fitted an ellipse to this set of points to obtain the HWHM of an inclined 2D Gaussian. The overall HWHM of the region is then given by the semi-major axis of the ellipse.