Context : Overall spherically symmetric , geometrically thin gas and dust shells have been found around a handful of asymptotic giant branch ( AGB ) carbon stars .
Their dynamical ages lie in the range of 10 ^ { 3 } to 10 ^ { 4 } years .
A tentative explanation for their existence is that they have formed as a consequence of mass-loss-rate modulations during a He-shell flash .

Aims : The detached shells carry information on their formation process , as well as on the small-scale structure of the circumstellar medium around AGB stars due to the absence of significant line-of-sight confusion .

Methods : The youngest detached shells , those around the carbon stars RÂ Scl and UÂ Cam , are studied here in great detail in scattered stellar light with the Advanced Survey Camera on the Hubble Space Telescope .
Quantitative results are derived assuming optically thin dust scattering .

Results : The detached dust shells around RÂ Scl and UÂ Cam are found to be consistent with an overall spherical symmetry .
They have radii of 19 \aas@@fstack { \prime \prime } 2 ( corresponding to a linear size of 8 \times 10 ^ { 16 } cm ) and 7 \aas@@fstack { \prime \prime } 7 ( 5 \times 10 ^ { 16 } cm ) , widths of 1 \aas@@fstack { \prime \prime } 2 ( 5 \times 10 ^ { 15 } cm ) and 0 \aas@@fstack { \prime \prime } 6 ( 4 \times 10 ^ { 15 } cm ) , and dust masses of 3 \times 10 ^ { -6 } and 3 \times 10 ^ { -7 } M _ { \odot } , respectively .
The dynamical ages of the RÂ Scl and UÂ Cam shells are estimated to be 1700 and 700 yr , respectively , and the shell widths correspond to time scales of 100 and 50 yr , respectively .
Small-scale structure in the form of less than arcsec-sized clumps is clearly seen in the images of the RÂ Scl shell .
Average clump dust masses are estimated to be about 2 \times 10 ^ { -9 } M _ { \odot } .
Comparisons with CO line interferometer data show that the dust and gas shells coincide spatially , within the errors ( \leq 1 \arcsec for UÂ Cam and \approx 2 \arcsec for RÂ Scl ) .

Conclusions : The results are consistent with the interpretation of geometrically thin gas and dust shells formed by a mass-loss eruption during a He-shell flash , and where interaction with a previous wind plays a role as well .
The mass loss responsible for the shells must have been remarkably isotropic , and , if wind interaction plays a role , this also applies to the mass loss prior to the eruption .
Clumpy structure is present in the RÂ Scl shell , possibly as a consequence of the mass loss itself , but more likely as a consequence of instabilities in the expanding shell .