Expected squared visibility of the limb darkened photosphere (blue solid line) as a function of the spatial frequency, along with the measured squared visibilities and related 1-σ error bars (one colour per wavelength). The thickness of the blue solid line corresponds to the 3-σ error related to the uncertainty on the stellar diameter. The best-fit model is represented by the dotted blue line with the residuals of the fit given in the middle panel. It corresponds to a limb-darkened photosphere of 0.736 ± 0.015 ± 0.012 mas in diameter surrounded by a uniform circumstellar emission of 1.37  ±  0.10  ±  0.13% in the H band. The bottom panel gives the residuals obtained by fitting only the stellar diameter (no circumstellar emission).