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Figure 1: VLTI ground baselines for Achernar observations and their corresponding projections onto the sky at different observing times. Left: Aerial view of VLTI ground baselines for the two pairs of 40 cm siderostats used for Achernar observations. Color magenta represents the 66 m (E0-G1; azimuth  , counted from North to East) and green the 140 m (B3-M0; ). Right: Corresponding baseline projections onto the sky ( ) as seen from the star. Note the very efficient Earth-rotation synthesis resulting in a nearly complete coverage in azimuth angles.

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Figure 2: Fit of an ellipse over the observed squared visibilities V2 translated to equivalent uniform disc angular diameters. Each V2 is plotted together with its symmetrical value in azimuth. Magenta points are for the 66 m baseline and green points are for the 140 m baseline. The fitted ellipse results in major axis milliarcsec, minor axis milliarcsec, and minor axis orientation (from North to East). The points distribution reveals an extremely oblate shape with a ratio .

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In the text

Figure 3: Comparison of ratios of squared visibility curves between the polar and equatorial directions . The black solid curve corresponds to  for the elliptical fit on Achernar's data, together with the corresponding uncertainties. Simply speaking  somehow reflects  , since interferometry is sensitive to the Fourier transform of the stellar brightness distribution. The colored region represents our attempt attain the black curve with our model for Achernar within the physically reasonable solutions A (orange; upper limit) and B (red; lower limit). This failure to reproduce the observations is a strong and direct indication that uniform rotation does not apply to rapidly rotating stars.

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In the text