1 Introduction

Be stars are non-supergiant B stars that at least once have displayed Balmer line emission. This occurs in about 20% of all B-type stars but, while this emission is thought to arise from a circumstellar disk, many details are unknown.

The early Be stars exhibit strongly variable winds evidenced by the rapidly variable UV resonance lines of highly ionized species, as well as spectral and photometric variations on timescales from hours to decades. The phases of emission in the optical and IR lines of hydrogen and several other species are called the Be phenomenon and most likely reflect changes in the structure of the circumstellar disk due to episodic ejections of mass.

The few interferometric observations of Be stars that exist (Stee & Bittar 2001; Quirrenbach et al. 1997) show clear evidence for a non-spherical or flattened envelope, often thought to be related to the usually high rotational velocities of these stars (Harmanec 1982; Marlborough 1986). The stellar rotation rates, however, are always lower than the critical velocities at which the centrifugal force balances gravitation at the equator. Thus, the centrifugal force by itself is inadequate to explain the formation of a disk around these stars. Note that, although high rotational velocities are usually observed for Be stars, this property is not part of the definition of a Be star (Collins 1987), and some of them rotate slowly (e.g. $\beta$ Cep, Donati et al. 2001; see also Mennickent et al. 1994).

The key problems in understanding this phenomenon are then: (i) how to enhance the specific angular momentum of material so that it can be ejected and attain a stable orbit, and (ii) how to eject quantities of mass in a nonsteady fashion.

NRPs and magnetic field processes have been proposed as possible mechanisms to explain the Be phenomenon (e.g. Osaki 1986; Underhill 1987). Multiperiodicity has already been detected in LPV of Be stars and has been generally attributed to NRPs (e.g. Gies 1994; Rivinius et al. 1998a, hereafter R1). Up to now, only $\beta$ Cep is known to host a magnetic field (Henrichs et al. 2000a).

Although $\omega$ Ori has a rich history with suspected pulsation and variable wind properties with a period close to 1 day, its behavior has remained a mystery. For these reasons, and because it can be observed from both hemispheres, it is a very suitable target for multi-site observations. Therefore a MuSiCoS campaign on this star was organized in 1998. In this paper, we first discuss the stellar parameters of $\omega$ Ori (Sect. 2) and recall the observational history of this star (Sect. 3). The data obtained during the campaign are presented in Sect. 4. In Sect. 5 we present a detailed analysis of the line-profile variations and give the frequencies found. The main frequency f₁ is studied in Sect. 6, while a second frequency f₂ is investigated in Sect. 7. Finally we discuss the obtained results (Sect. 8) and present concluding remarks (Sect. 9).