¹ Observatoire de la Côte d'Azur, Département Fresnel, CNRS UMR F-06460 Saint Vallier de Thiey, France
² Observatoire de Grenoble, BP53X, 38041 Grenoble cedex 9, France
³ Department of Astronomy, Kyoto University, Kyoto, 606-01, Japan

We present spectrally resolved interferometry of the Be star  Cas in '88, '91,'93 and '94, obtained with the GI2T interferometer. The analysis of high spatial resolution data across the H line reveals azimuthally asymmetric variations which are correlated with those of V/R of the H profile. This correlation supports a prograde one-armed oscillation precessing in the equatorial disk of  Cas due to the confinement by a radiative effect. We examine the occurence of such oscillations in the context of the latitude dependent radiative wind model developed for previous GI2T interferometric observations of this star. We find that this enhanced equatorial density pattern may be located at 1.5 stellar radii from the stellar surface. We follow its possible rotation through the -99 km s^-1, +92 km s^-1, +140 km s^-1 and +41 km s^-1 iso-velocity regions which provides us with the approximate stellar longitudes: 224°, 42°, 153°, and 184° for '88, '91, '93, '94 epochs respectively. Thus,  Cas is the second Be star after  Tau for which interferometric observations directly evidence a prograde one-armed oscillation of its equatorial disk.

1999, A&A, 345, 203