Interferometric insight into gamma Cassiopeiae long-term variability

We present spectrally resolved interferometry of the Be star gamma Cas in ' 88, '91, '93 and '94, obtained with the GI2T interferometer. The analysis o f high spatial resolution data across the H alpha line reveals azimuthally asymmetric variations which are correlated with those of V/R of the H alpha profile. This correlation supports a prograde one-armed oscillation preces sing in the equatorial disk of gamma Cas due to the confinement by a radiat ive effect. We examine the occurrence of such oscillations in the context o f the latitude dependent radiative wind model developed for previous GI2T i nterferometric observations of this star. We find that this enhanced equato rial density pattern may be located at 1.5 stellar radii from the stellar s urface. 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 results in a pproximate stellar longitudes: 224 degrees, 42 degrees, 153,degrees and 184 degrees for '88,'91,'93,'94 epochs respectively. Thus, gamma Cas is the se cond Be star after zeta Tau for which interferometric observations directly evidence a prograde one-armed oscillations of its equatorial disk.