  Aims. We aim to resolve the spatial and kinematic sub-structures in five
detached-shell sources to provide detailed constraints for hydrodynamic models
that describe the formation and evolution of the shells. Methods. We use
observations of the 12 CO (1-0) emission towards five carbon-AGB stars with
ALMA. The data have angular resolutions of 0.3 arcsec to 1arcsec and a velocity
resolution of 0.3 km/s . This enables us to quantify spatial and kinematic
structures in the shells. Results. The observed emission is separated into two
distinct components: a more coherent, bright outer shell and a more
filamentary, fainter inner shell. The kinematic information shows that the
inner sub-shells move at a higher velocity relative to the outer sub-shells.
The observed sub-structures confirm the predictions from hydrodynamical models.
However, the models do not predict a double-shell structure, and the CO
emission likely only traces the inner and outer edges of the shell, implying a
lack of CO in the middle layers of the detached shell. Previous estimates of
the masses and temperatures are consistent with originating mainly from the
brighter subshell, but the total shell masses are likely lower limits.
Conclusions. The observed spatial and kinematical splittings of the shells
appear consistent with results from hydrodynamical models, provided the CO
emission does not trace the H2 density distribution in the shell but rather
traces the edges of the shells. It is therefore not possible to constrain the
total shell mass based on the CO observations alone. Complementary observations
of, e.g., CI as a dissociation product of CO would be necessary to understand
the distribution of CO compared to H2.
