Top: dependence of simulated differential phases φ_diff on R_eq and on d (solid line: tested model; dashed line: reference model as described in Sect. 2). The parameters of the tested model are identical to those of the reference model, except that R_eq = 9 R_⊙ (left) and d = 60 pc (right). The values of the tested and reference models differ by  ≃20%; they are indicated in the upper left part of the figures. All φ_diff models were calculated across the Br γ line with a spectral resolution of 12 000 at four projected baselines (B_proj = 75 m and 150 m, and PA = 45° and 90°), which are typical values attained with VLTI/AMBER. These wavelengths and baselines result in visibility amplitudes between 0.6 and 0.9 for the studied stellar models, corresponding to a partially resolved star. The horizontal dashed gray lines indicate the typical error bar of AMBER φ_diff (σ_φ =  ± 0.5°). The φ_diff signature of stellar rotation appears as an s-shaped curve well visible inside Br γ, with an amplitude higher than σ_φ at all baselines. Bottom: difference between φ_diff of the tested and reference models (solid minus dashed curves in the top panel). The continuum of all curves is equal to zero, but they were shifted for better readability.