SiO emission from a huge, detached shell in IRC+10420

We present observations of the upsilon = 0 J = 2-1 thermal emission of SiO from the very luminous evolved star IRC+10420, performed with the Plateau d e Bure interferometer. Our high-resolution maps show that the emission come s from a huge hollow shell, with a typical radius of similar to 10(17) cm, that is expanding at similar to 35 km s(-1). This surprising result is stre ngthened by model fitting of the flux distribution, that explains the obser vations assuming that the width of the shell is not larger than half its ty pical radius. The origin of this feature is discussed.