Polarimetry of evolved stars

We present broadband optical polarimetry, and broadband optical and infrared photometry, of eight RV Tau-type and five R CrB-type stars; much of the photometry and polarimetry was obtained simultaneously. For nine of the objects polarimetric data is reported for the first time. We have estimated and subtracted the interstellar component of polarization, allowing us to determine the level of intrinsic polarization. In some cases this is $\simeq$1%–2% even when the star is in a bright photometric state. We consider this to be evidence for the presence of permanent clumpy non-spherical dust shells around the RV Tau and R CrB-type stars we observed. Our polarimetric and photometric data lead us to conclude that, for most of our programme stars, neutral extinction must be significant in their circumstellar envelopes. Apart from the brightness variations due to pulsations and changes in the effective temperature of stars, there is clear evidence of wavelength-independent flux variations – with amplitude from $0\fm 5$ to $1\fm 0$ – implying the presence of large ($a\ga 0.15$$\,\mu$m) dust particles. Rapid (~2 hours) evolution of the infrared flux distribution at the level of ~$0\fm 6$ in the JHKL bands was detected in the RV Tau star R Sct.