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