Short period variable stars. III. High resolution spectra of the K line in gamma Bootis and three other A type stars.

Fine variations in the K line profile of y Boo (HD 127 762, A7 III), are presented from a series of high resolution spectra (2.2 A/mm) obtained with an electronic camera during one night. Differential radial velocities have been measured on each profile. The velocity of the line axis, defined at half intensity, varies in a quasi periodical way. Correlative variations with time of the equivalent width and the half width are clearly shown. Two Am and one Ap star have also been observed in the same line; no reversal can be detected. A photospheric model is derived for Boo. We suggest two interpretations for the K line profile: chromospheric temperature rise plus velocity field, or Hillendahl mechanism. Key words: short period variable stars - K line in A stars - line profile variations - radial velocity variations On de faibles variations du profil de la raie K de y Boo (HD 127 762, A 7 III) observ6es sur `me s6rie de spectres a' haute resolution (2,2 A/mm) obtenus au cours d'une meme nuit avec `me 6lectronique. Des vitesSes radiales ont t sur chaque profil. La vitesse de l'axe de la raie, a' mi-hauteur, varie d'une quasi- . Des variations es dans le temps de la largeur qnivalente et de la largeur mi-hauteur sont clairement mises en vidence. Deux toiles Am et `me Ap ont aussi 6t6 dans la me me raie; on ne aucun renversement. Un de est propos6 pour Boo. On deux pour le profil de la raie K: remont6c de temp6rature et champ de vitesse, ou m6canisme Hillendahl.