  Stars on the asymptotic giant branch (AGB) lose considerable amounts of
matter through their dust-driven stellar winds. A number of such sources have
been imaged by Herschel/PACS, revealing a diverse sample of different
morphological types. Among them are a few examples which show geometrically
thin, spherically symmetric shells which can be used to probe the mass loss
history of their host stars. We aim to determine the physical properties of the
dust envelope around the two carbon stars U Hya and W Ori. With the
much-improved spatial constraints from the new far-infrared maps, our primary
goal is to measure the dust masses contained in the shells and see how they fit
the proposed scenarios of shell formation. We calculated the radiative transfer
of the circumstellar dust envelope using the 1D code More of DUSTY (MoD).
Adopting a parametrised density profile, we obtained a best-fit model in terms
of the photometric and spectroscopic data, as well as a radial intensity
profile based on Herschel/PACS data. For the case of U Hya, we also computed a
grid of circumstellar envelopes by means of a stationary wind code and compare
the results of the two modelling approaches. The Herschel/PACS maps show U Hya
surrounded by a detached shell of $114''\ (0.12\,\mathrm{pc})$ in radius,
confirming the observations from previous space missions. The dust masses
calculated for the shell by the two approaches are consistent with respect to
the adopted dust grain properties. In addition, around W Ori, we detect for the
first time a weak spherically symmetric structure with a radius of $92''\
(0.17\,\mathrm{pc})$ and a dust mass of
$(3.5\pm0.3)\times10^{-6}\,\mathrm{M_\odot}$.
