Mode-switching timescales in the classical variable stars

Near the edges of the instability strip, the rate of stellar evolution is larger than the growth rate of the pulsation amplitude, and the same holds whenever the star is engaged in pulsational mode switching. Stellar evolution therefore controls the onset of pulsation at the edges of the instability strip and of mode switching inside it. Two types of switchings (bifurcations) occur. In a soft bifurcation, the switching timescale is the inverse harmonic mean of the pulsational modal growth rate and of the stellar evolution rate. In a hard bifurcation, the switching times can be substantially longer than the thermal timescale, which is typically of the order of 100 periods for Cepheids and RR Lyrae stars. We discuss some of the observational consequences, in particular the paucity of low-amplitude pulsators at the edges of the instability strip.