¹ Department of Physics, Catholic University of America, Washington, DC 20064, USA; Present address: Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218
² National Research Council of Canada, Herzberg Institute of Astrophysics, 5071 W. Saanich Rd., Victoria, BC Canada V9E 2E7

Recently Neiner et al. reported that the B2 IV-V star  Cas contains a weak magnetic field which varies on the same 5.37 day period found from the modulations of its N V, C IV, and Si IV UV resonance lines. We have studied the time variable properties of the same resonance lines in greater detail to determine the physical characteristics of the magnetospheric structure responsible for them. In our formulation this structure takes the form of an axisymmetric "disk" similar to those around magnetic He-strong Bp stars. This structure corotates with the star, covering greater or lesser amounts of its area during its transit.  Cas offers a special case because we observe it from a low inclination and yet its magnetic axis is substantially inclined to the rotation axis. The equivalent width-phase curves show a flat maximum for half the cycle, indicating that the disk is extended out of the plane, extends to the star's surface in the magnetic plane, or both. Synthetic spectra of the line profiles during the maximum and minimum occultation phases can be best reconciled with a disk geometry in which the resonance lines are formed at a closed outer edge and along a thin outer layer. We speculate that observed weak redshifted emission is formed in "auroral caps" located near the magnetic poles of the star. We argue that this results from shocks of stagnated wind material returning to the star and shocking against the outflowing wind.

Accepted by A&A