Nonequilibrium effects on the Cepheids between the gas and radiation are investigated. It is found that the gas and radiation all contribute to the driving of the pulsation. The nature of the excitation, though, is determined by the modulation and frozen temperature mechanism of the gas. The red edge of the instability strip is surprisingly obtained in the frozen convection approximation. It is a result of the interaction between the modulation and frozen temperature mechanism. The shapes of the instability strips are narrow at the top and wide at the bottom, and cut off at sufficiently high luminosities. This is a result of the mode selection effect produced by the strong nonadiabaticity of the pulsation.
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