Conclusion

What factors do determine the pulsation characteristics, i.e. amplitudes and modes of pulsation in the $\delta$ Scuti instability strip? This question cannot be addressed on the basis of a single case. But it could be approached as illustrated here. The detailed investigation of differences in variability behaviour between two components of a binary system or of a common origin pair may provide relevant clues: differences in origin and age can be ruled out as also differences in overall chemical composition. In this example it is easy to identify which component is the short-period pulsating star and the information obtained from the astrometry can be coupled to the astrophysical parameters of the individual components. Even better would be to investigate these characteristics in a close visual binary for which information on the orbital motion can also be derived. This would allow to obtain a direct estimation of the stellar mass, independent from the choice of modelisation. The derivation of the pulsation constant would be more straightforward (the error on the mass defines the accuracy of Q). More cases like this one should be looked into.
With this application in mind, we have made a crossidentification between the Annex of Variable Stars and the Annex of Double and Multiple Stars from the Hipparcos Catalogue (ESA, 1997). Some 2500 systems with at least one variable component have been identified. But the description of the variability or the light curve in the Annex always refer to the combined magnitudes. Additional observations should help identify which component is variable and which of these are binary systems that offer the opportunity of coupling the information obtained by astrometric means (association type - parallax - total mass) to the astrophysical quantities (luminosity ratio - colours - pulsation characteristics) in order to obtain a consistent picture of the system and its components.
Acknowledgements We thank the Geneva team (Dr. G. Burki) for the telescope time put at our disposal in June 1990 and September 1991. Dr. Sperl is kindly acknowledged for making the programme Period98 available for this application. We appreciate the help of Dr. L. Eyer in the selection of the Hipparcos Epoch Photometry data.

References Bloomfield, P., 1976, John Wiley & Sons, New-York, Sect. 5.2 Duflot M., Figon P. and Meyssonnier N., 1995, A&AS 114, 269 ESA, 1997, The Hipparcos and Tycho Catalogues, ESA SP-1200 Flower, Ph., 1996, ApJ 469, 355 Gray, R.O. and Garrison, R.F., 1989, ApJS 69, 301-321 Kobi, D. and North, P.,1990, A&AS 85, 999-1014 Künzli, M., North, P., Kurucz, R.L. and Nicolet, B., 1997, A&AS 122, 51-77 Lampens, P., 1992, Delta Scuti Newsletter 5, 9-10 Levato, A., 1975, A&AS 19, 91-99 North, P. ,1996, private communication Rufener, F., 1988, Catalogue of stars measured in the Geneva Observatory Photometric System. 4th edition, ed. Geneva Observatory Schaller, G., Schaerer, D., Meynet, G., and Maeder, A., 1992, A&AS 96, 269-331 Sperl, M., 1998, Manual for Period98 : V1.0.4 A period search-program for Windows and Unix, (http://dsn.astro.univie.ac.at/period98) Tsvetkov, Ts.G., 1993, Ap&SS 208, 285-290