This work was supported by the research grant No. 2 P03D 001 08 from the Polish Scientific Research Committee (KBN). Stellar models and linear nonadiabatic pulsation data were obtained from Prof. W. Dziembowski and Dr. A. Pamyatnykh. ESA IUE Observatory at VILSPA provided the IUE tapes. To them all, we express our thanks. We also would like to thank the referees for careful reading of the manuscript and for valuable comments. The editorial office kindly made english corrections.
© European Southern Observatory (ESO) 1997
Online publication: October 15, 1997