Proper motions of dust shells surrounding NML Cygni

The distribution of dust emitted by the supergiant star NML Cygni has been resolved by interferometry at 11 mum wavelengths at various times over a period of 6 yr. Results show there are two discrete dust shells, which have both moved away from the star approximately the same amount during the 6 yr period. This allows determination of the time between ejection of material forming the two shells to be 65 +/- 14 yr. Assuming the radial outflow velocity can be derived from Doppler-measured velocities of masers surrounding the star, its distance can be calculated from the observed angular motion to be 1220 +/- 300 pc. This decreases the luminosity of the star by about 1 mag over that deduced from the distance 1900 pc previously assumed.