Expected squared visibility as a function of the baseline length for Vega’s limb darkened photosphere (blue solid line), given with the measured squared visibilities and related error bars (blue symbols for June 9 and 10, green symbols for June 13 and 14 on polarisation 1, and red symbols for June 13 and 14 on polarisation 2). The thickness of the blue solid line corresponds to the 5-σ uncertainty related to the uncertainty on the stellar diameter. The best-fitted models of a limb-darkened photosphere surrounded by a uniform circumstellar emission (dotted blue line), a zodiacal disc (dashed green line), and a narrow ring (dash-dotted red line) are also represented (see discussion in Sect. 3). Residuals of the fit are given in the bottom panel for the uniform disc model.