THE SHOCK-WAVE PROPAGATION EFFECTS IN BL HERCULIS .1. THE H-ALPHA PROFILE

For the first time, high spectral resolution observations of the Ha pr ofile of BL Her, obtained with a modem detector, during the whole puls ating period are presented. It clearly appears that the profile evolut ion is very similar to that observed in RR Lyrae stars which are local ised just below the BL Herculis stars in the Population III instabilit y strip. No secondary emission is detected. The pulsation has the form of a running wave. The shock wave associated with the emission is acc elerated during its propagation between the photosphere and the region where the hydrogen line is formed. The effect of the 2:1 resonance be tween the fundamental mode and the second overtone is well observed on the photospheric radius and acceleration curves.