We present the discovery of a huge ( 19 \arcmin \times 16 \arcmin ) dust ring surrounding a bright ( V = 10.60 ) red star .
The dust ring has , at D = 700 pc , a diameter of 4 pc , and a central hole of \sim 1.5 pc across .
Part of the shell is also seen as an absorption nebulosity .
The star is classified as a M3III AGB star .
Among AGB stars its detached shell is of unrivalled size .
Detached shells around AGB stars are normally interpreted in terms of thermal pulses .
However , in this case a significant fraction of the shell may consist of swept-up ISM ; the detached appearance can be explained with wind–ISM interaction .
We present a model where the AGB wind has been stopped by the surrounding ISM , and the swept-up shell is now expanding at the sound speed .
The model predicts that the ring will disperse over a few times 10 ^ { 5 } yr , and eventually will leave a large hole in the ISM surrounding the AGB star or its future planetary nebula .