7 Conclusions

We have shown that the primary of the close binary $\psi ^2$ Ori exhibits non-radial pulsations. From our time series of spectra we have derived two pulsation frequencies of modes with intermediate $\ell$ values.

Following the frequency considerations in the previous sections, we conclude that the observed frequencies f₁=10.48c/d and f₂=10.73c/d are consistent with internally excited p-mode $\beta$ Cephei oscillations, and that it is unlikely that tidal forcing plays a dominant role for these modes. We stress, however, that more data is needed to confirm the pulsation frequencies, from which the above results were derived.

The observed ellipsoidal variations in $\psi ^2$ Ori must be due to deformations due to equilibrium tide or dynamical tide with $\ell=2$ behaviour. These should lead to apparent frequencies in the line-profile time series of low multiples of the orbital frequency. However, as indicated in Sect. 5, the fact that the line profiles of the two binary components cross also gives rise to orbital harmonics in the profile series. In our analysis it is difficult to disentangle these from the effect of tidal deformation. In order to investigate such deformations spectroscopically, one may try a dedicated technique to disentangle the spectra of double-lined spectroscopic binaries (Hadrava 1995) before attempting a frequency analysis on the line-profile variability.

Waelkens & Rufener (1983) suggested that for the closest binaries tidal interactions have a damping effect on $\beta$ Cephei oscillations. The case of $\psi ^2$ Ori provides evidence that this suggestion is not always true.

Acknowledgements

The authors wish to acknowledge the referee, Dr. C. Aerts, and Dr. B. Willems for their valuable comments.

J.A. would like to acknowledge John Telting for being a great supervisor and a good friend. He would also like to thank Johan Knapen for arranging the placement year at the ING, and Dr. A. Batten for his remarks regarding his work on the apsidal motion of $\psi ^2$Ori. Financial support is acknowledged from the Isaac Newton Group of Telescopes.

C.S. thanks John Telting and Saskia Prins for their hospitality in the days prior to the observing run.