Synthetic interferometric measurements and modelling across the rotational phase of the system. Top: integrated images over the Brγ line. Green (uniform disc) and yellow (Gaussian disc) ellipses are the characteristic sizes measured with a GRAVITY-like instrument. Middle-top: pure line visibility amplitude observables associated with the corresponding models. The visibility variation (for a given baseline) as the uv plane rotates is the specific signature of an elongated object. Middle-bottom: pure phase visibility across the line profile for the six baselines of the VLTI. Colours encode the observing time. Bottom: velocity map of the radiative transfer model. The coloured dots represent the measurement of the photo-centre derived from the phase visibility in each available spectral channel (see Appendix B). In each figure, the magnetosphere and the stellar surface have been normalised independently for display purposes.