Imaging of detached shells around the carbon stars R Scl and U Ant throughscattered stellar light

We present the first optical images of scattered light from large, detached gas/dust shells around two carbon stars, R Scl and U Ant, obtained in narr ow band filters centred on the resonance lines of neutral K and Na, and in a Stromgren b filter (only U Ant). They confirm results obtained in CO radi o line observations, but also reveal new and interesting structures. Toward s R Scl the scattering appears optically thick in both the K and Na filters , and both images outline almost perfectly circular disks with essentially uniform intensity out to a sharp outer radius of approximate to 21". These disks are larger - by about a factor of two - than the radius of the detach ed shell which has been marginally resolved in CO radio line data. In U Ant the scattering in the K filter appears to be, at least partially, opticall y thin, and the image is consistent with scattering in a geometrically thin (approximate to3") shell (radius approximate to 43") with an overall spher ical symmetry. The size of this shell agrees very well with that of the det ached shell seen in CO radio line emission. The scattering in the Na filter appears more optically thick, and the image suggests the presence of at le ast one, possibly two, shells inside the 43" shell. There is no evidence fo r such a multiple-shell structure in the CO data, but this can be due to co nsiderably lower masses for these inner shells. Weak scattering appears als o in a shell which is located outside the 43" shell. The present data do no t allow us to conclusively identify the scattering agent, but we argue that most of the emission in the K and Na filter images is to due to resonance line scattering, and that there is also a weaker contribution from dust sca ttering in the U Ant data. Awaiting new observational data, our interpretat ion must be regarded as tentative.