SIMBAD references

For the first time, high spectral resolution observations of the Hα profile of BL Her, obtained with a modern detector, during the whole pulsating period are presented. It clearly appears that the profile evolution is very similar to that observed in RR Lyrae stars which are localised just below the BL Herculis stars in the Population II instability strip. No secondary emission is detected. The pulsation has the form of a running wave. The shock wave associated with the emission is accelerated during its propagation between the photosphere and the region where the hydrogen line is formed. The effect of the 2:1 resonance between the fundamental mode and the second overtone is well observed on the photospheric radius and acceleration curves.