Theoretical squared visibilities of a star without a spot (solid black line), and of a star with a spot (ws) as modelised with COMETS (Ligi et al. 2015). The different solid color lines represent the squared visibilities at different wavelengths and positions. The black circles represent the actual interferometric measurements (with error bars) of GJ504 performed with VEGA/CHARA. Left panel: a spot with a filling factor p = 7%. Right panel: the same but for p = 22%.