ON THE ORIGIN OF THE ALTERNATING DEEP AND SHALLOW LIGHT MINIMA IN RV TAURI STARS - R-SCUTI AND AC HERCULIS

Clean power spectra of the two well-known RV Tauri stars: R Scuti and AC Herculis are presented. R Sct is one of the most irregular stars of this pulsating variable class, while AC Her is one of the most regular. Except the frequency peaks at nu(0)/2 and 3-nu(0)/2, where the fundamental period P0 = 1/nu(0) is assumed to be the interval between two successive shallow and deep light minima, none of the higher subharmonics or their multiples are detected. Moreover, for R Sct, each frequency peak shows a multi-component structure which is not encountered within power spectra deduced from hydrodynamical stellar models. This means that the pulsation of R Sct is never strictly periodic. Thus, both the period-doubling bifurcation scenario suggested by Kovacs & Buchler (1988) and the type-I intermittency occurring just after a 4-cycle period-doubling bifurcation (Aikawa 1990) are clearly not confirmed. The origin of the characteristic alternation of shallow and deep minima is qualitatively consistent with a radiative shock wave effect associated with the development of an extended pulsation running wave. This suggestion merits a realistic but difficult theoretical investigation.