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Context. RV Tauri-type variables are pulsating post-AGB stars that evolverapidly through the instability strip after leaving the Asymptotic GiantBranch. Their light variability is dominated by radial pulsations. Members ofthe RVb subclass show an additional variability in form of a long-termmodulation of the mean brightness, for which the most popular theories allassume binarity and some kind of circumstellar dust. Here we address if theamplitude modulations are consistent with the dust obscuration model. Aims. Wemeasure and interpret the overall changes of the mean amplitude of thepulsations along the RVb variability. Methods. We compiled long-termphotometric data for RVb-type stars, including visual observations of theAmerican Association of Variable Star Observers, ground-based CCD photometryfrom the OGLE and ASAS projects and ultra-precise space photometry of one star,DF Cygni, from the Kepler space telescope. After converting all theobservations to flux units, we measure the cycle-to-cycle variations of thepulsation amplitude and correlate them to the actual mean fluxes. Results. Wefind a surprisingly uniform correlation between the pulsation amplitude and themean flux: they scale linearly with each other for a wide range of fluxes andamplitudes. It means that the pulsation amplitude actually remains constantwhen measured relative to the system flux level. The apparent amplitudedecrease in the faint states has long been noted in the literature but it wasalways claimed to be difficult to explain with the actual models of the RVbphenomenon. Here we show that when fluxes are used instead of magnitudes, theamplitude attenuation is naturally explained by periodic obscuration from alarge opaque screen, one most likely corresponding to a circumbinary dusty diskthat surrounds the whole system.

Amplitude variations of modulated RV Tauri stars support the dust obscuration model of the RVb phenomenon