¹ Laboratoire Univ. d'Astroph. de Nice (LUAN), CNRS UMR 6525, Parc Valrose, 06108 Nice Cedex 02, France
² European Southern Observatory (ESO), Alonso de Cordova 3107, Casilla 19001, Vitacura, Santiago 19, Chile
³ Observatoire de la Côte d'Azur, Département FRESNEL, CNRS UMR 6528, Boulevard de l'Observatoire, B.P 4229, 06304 Nice, France
⁴ European Southern Observatory (ESO), Karl-Schwarzschild str. 2, D-85748 Garching, Germany

We report here the first observations of a rapidly rotating Be star,  Eridani, using Earth-rotation synthesis on the Very Large Telescope (VLT) Interferometer. Our measures correspond to a 2a/2b=1.56±0.05 apparent oblate star, 2a and 2b being the equivalent uniform disc angular diameters in the equatorial and polar direction. Considering the presence of a circumstellar envelope (CSE) we argue that our measurement corresponds to a truly distorted star since  Eridani exhibited negligible H  emission during the interferometric observations. In this framework we conclude that the commonly adopted Roche approximation (uniform rotation and centrally condensed mass) should not apply to  Eridani. This result opens new perspectives to basic astrophysical problems, such as rotationally enhanced mass loss and internal angular momentum distribution. In addition to its intimate relation with magnetism and pulsation, rapid rotation thus provides a key to the Be phenomenon: one of the outstanding non-resolved problems in stellar physics.

Accepted by A&A (Letters)
Preprints on the web at http://arxiv.org/abs/astro-ph/0306277