Observations of parametric resonance in AH Camelopardalis, RR Lyrae type stars with the Blazhko effect

Blazhko effect with the period 11d,0±0d,2 is detected in AH Cam by photoelectric B and V observations. The frequency analysis has shown the secondary wave with the period of 0d.18128 in the light curve, it is approximately twice shorter than the basic one 0d,36873. The secondary wave amplitude depends strongly on the basic period phase in which its maximum is located, and reaches 0m,7 B when its maximum coincides with the ascending branch of the basic wave. Complex interaction of the two waves in the shell of the star is interpreted as a parametric resonance excitation of the 3rd overtone motion. Analogous resonance interaction of the fundamental and the first overtone is found in the light curve of the double mode RR Lyrae type star V68 in the globular cluster M3. It suggests that the Blazhko effect and double mode pulsations are principally the same phenomenon, but arising between different eigenfrequencies of the star shells