5 Discussion

The $\delta$ Scuti star, 57 Tau, seems to be unique in many aspects. The extremely small amplitude of the excited p-modes has been first mentioned by B.J. McNamara and is confirmed by the present study. The amplitude of the dominant mode excited in 57 Tau is only about twice as the residual amplitude level of $\theta$ Tucanae prewhitened with 13 frequencies (Paparó et al. [1996]). We suggest two possible explanations for the periodicities in the low frequency domain. The more conventional, although non-trivial, explanation relates to the geometry of the system, i.e., a binary or multiple nature of 57 Tau. Spectroscopic observations of 57 Tau are urgently needed to check this explanation. The less conventional but more exciting explanation is that they represent the excitation of g modes as well as the p modes. According to its spectral type, 57 Tau is situated on HR diagram near to the cool border of the instability strip, exactly in the region where $\gamma$ Dor stars are located. The g-mode frequencies of $\gamma$ Dor stars discovered in the last few years (Abt et al. [1983]; Antonello & Mantegazza [1986]; Krisciunas & Guinan [1990]; Balona et al. [1994]; Breger et al. [1997]; Zerbi et al. [1997]; Kaye et al. [1998]; Kaye et al. [1999a]) occur in the same frequency range as in the low frequency domain of 57 Tau's amplitude spectrum. If this interpretation is correct 57 Tau would be the first $\delta$ Scuti type star where both p & high overtone g modes are excited, i.e. where both $\delta$ Scuti-type and $\gamma$ Dor-type pulsation are found in the same star. This would present a great challenge to theoreticians. If the mass ratio were obtained from the binary (or multiple) system, and the chemical composition & distance based on the membership of Hyades are assumed, 57 Tau is a good candidate for modelling. The possible excitation of the frequency at 29.83 c/d with variable amplitude and asymmetric shape of some single unusual cycles, might require non-linear and/or nonstationary processes.

High resolution, high SNR spectra of 57 Tau have been recently obtained by Kaye et al. ([1999b]). Subsequent analysis by Kaye ([1999c]) indicates that 57 Tau is a spectroscopic binary and some of the low frequency variations mentioned in the present paper "are probably due to duplicity (i.e. geometric and proximity) effects''. Further multi-site observations of 57 Tau by the $\gamma$ Dor network are also recommended.

Acknowledgements

Hungarian research grants, OTKA Nos. T 021074 and T 025288, are acknowledged. MP acknowledges to the Hungarian Academy of Sciences and the Spanish Scientific Council (CSIC) for support of her visit to Spain. The hospitality of staff members in Granada (IAA) is greatly appreciated. ER and AR acknowledge to DGES for support under project PB 96-0840. The project was supported by the Chinese National Natural Science Foundation. The authors are indebted to Dr. E. Poretti for his referee report.