1 Introduction

High-resolution, high signal-to-noise spectroscopic observations have shown that the presence of multiperiodic variability in photospheric lines is rather common among hot stars of luminosity class V-III (Kambe et al. 1990; Floquet et al. 1992, 1996; Gies & Kullavanijaya 1988). Spectroscopic observations of rapid rotators (e.g., Be stars) with sufficient temporal resolution have shown that the observed line profile variability (lpv) is characterized by the presence of "moving bumps" travelling from blue to red across the line profile in timescales of $\sim$hours. The prototype object showing this kind of behavior is the Be star $\zeta$Ophiuchi. The phenomenon has also been observed in O stars and $\delta$Scuti variables (Kennelly et al. 1992, hereafter KWM) among others. Non-radial pulsations (NRP) have been proposed as an explanation for the lpv observed in hot stars (e.g., Smith 1977; Gies 1991 and references therein). NRP produce lpv thanks to the combination of the Doppler displacement of stellar surface elements with their associated temperature variations due to the compression/expansion caused by the passage of the waves through the photosphere. In favorable cases these variations can also be detected photometrically. In rapid rotators, there is a one-to-one correlation between the points in the broaded line profile and the position of inhomogeneities on the stellar surface (the so-called "Doppler Imaging"). The additional velocity fields due to NRP produce then series of "bumps" that will move from blue to red across the line profile due to the stellar rotation and wave motion (Vogt & Penrod 1983).

Be stars are rapid rotators of spectral type late O to early A and luminosity class V-III, showing a near infrared excess and Balmer emission lines imputed to an equatorially concentrated circumstellar envelope, produced by sporadic mass ejection episodes. The causes of the "abnormal" mass loss are as yet unknown. In spite of their high $V\sin i$, rapid rotation alone cannot explain ejection episodes as Be stars are not observed to rotate sufficiently near the break-up velocity. NRP could be the clue to the Be phenomenon, providing the surplus of perturbation that is needed to cause mass ejection. Thanks to recent theoretical models that explain the excitation of NRP in hot stars through opacity mechanisms (Dziembowski & Pamyatnykh 1991, 1993; Gautschy & Saio 1993), the idea that lpv in B-Be stars can be caused by NRP has been reinforced. In fact, the presence of NRP in B-Be stars (the so-called "Slowly-Pulsating Variables" - SPB) is a logical issue in the sense that it bridges the gap along the main sequence between $\beta$Cephei and $\delta$Scuti pulsating variables (Dziembowski 1997). In this paper, we present the results of an extensive search for periodic oscillations in the $\zeta$Oph-type Be star $\eta$Cen. The observational material consists of spectroscopic and photometric observations conducted at Laboratório Nacional de Astrofísica (LNA) (Pico dos Dias, MG-Brazil). Frequency analysis has been performed by means of algorithms employing Fourier techniques. Photometric and spectroscopic data of Cuypers et al. (1989) and Stefl et al. (1995, hereafter SBHB) and photometric observations obtained with the Hipparcos satellite are also reconsidered.

$\eta$Cen (HD 127972; HR 5440; SAO 225044; MWC 232) is a rapidly-rotating ($V\sin i\simeq 360$ km s^-1), bright (V$\simeq$2.3) early-type Be/shell star. Cuypers et al. (1989) and Cuypers (1991) found triple-wave and double-wave photometric modulations with periods 1.92 d and 1.28 d, respectively. Bumps moving across the HeI 667.8 nm profile detected by Leister et al. (1994) were attributed to a NRP mode with $\mid$m $\mid$=l $\simeq$ 15 or $\mid$m$\mid$$\approx$6, l $\approx$ 7. Variability in both lpv (visible) and UV flux with a timescale of 1.64 c/d was detected by Gies (1994). Stefl et al. (1995) found nearly sinusoidal brightness (Strömgren system) and radial velocity variations (SiIII $\lambda$455.26 nm line) with period p=0.64 d (1.56 c/d). Finally, the Hipparcos catalogue (ESA 1997) quotes for the star a photometric period of 1.28 d ($\equiv$0.78 c/d).