PULSATING AND NONPULSATING STARS IN HYADES OBSERVED BY HIPPARCOS SATELLITE

A study of the stars in the instability strip of Hyades cluster has be en performed taking into account the new estimate of the stellar paral laxes. For taking advantage of the improved accuracy of the distances and hence absolute magnitudes it is necessary to separate the photomet ric effects of close companions, which appear quite common in Hyades. The delta Scuti stars, differently from nonpulsating stars, appear to be located also above the main sequence, and this is due probably to t heir large rotational velocity. The comparison of the rotational effec ts on the star position in the color-magnitude diagram with theoretica l model predictions tends to confirm the importance of differential ro tation. A brief discussion of theta(2) Tau and some remarks on the X-r ay emission of other two delta Scuti stars are also reported.