5 Summary and conclusions

In this paper, we analysed new photometric and spectroscopic data of $\eta$Centauri obtained in 1993 and 1995, observations published by SBHB and Cuypers et al. (1989) and Hipparcos photometric data (1997). A search for periodicities was performed with the help of CLEAN, CLEANEST and the Fourier Doppler Imaging (FDI) methods. Multiperiodic variability seems to be definitively established for this rapidly rotating Be star, and we interpret it in terms of non-radial pulsations. Indeed, the pulsation theory predicts the occurrence of unstable modes in B stars, excited by the metal opacity mechanism.

The star shows amplitude and frequency variability in time scales of years (e.g. before and after early 1993), days and hours; they are naturally explained by beating or resonant coupling phenomena.

Night-to-night variations detected during the 8-d observing period are characterized by alternately dominant l=2 and l=4 modes. This could be due to a beat modulation with the bump superperiod or with the stellar rotation.

The main frequencies found in our analyses (and the associated l values) are: 1.29 c/d (l=2), 1.78 c/d (l=1-3), 3.82 c/d (l=2), 4.34 c/d (l=3) and 4.51 c/d (l=4). Those with high amplitude and longer periods can be attributed to high-order g-modes, but less intense, short-term ($\sim$hours) periodicities have also been detected that can be attributed to p-modes. $\eta$Cen could be thus one of those "hybrid" stars, lying in the HR diagram at the border region between $\beta$Cep stars (p pulsators) and the "classical" SPB variables (g pulsators), showing both p- and g- modes.

Acknowledgements

We thank the referee, Dr. D.R. Gies for judicious comments that significantly improved the paper. The authors thank A.D. de Souza Jr. for helping with photometric data reductions. This research was partially supported by Fapesp (grant No. 95/2719-2), CNRS and CNPq (research grants Nos. 301050/88-RN, 910316/92 and 910180/96-2). S.J. acknowledges financial support by CNPq and Fapesp through grants No. 301465/95-9.