| Distances to AGB stars with optically thick circumstellar shells cannot be determined using optical parallaxes. However, for stars with OH 1612 MHz maser emission emanating from their circumstellar shells, distances can be determined by the phase-lag method. This method combines a linear diameter obtained from a phase-lag measurement with an angular diameter obtained from interferometry.
The phase-lag of the variable emission from the back and front sides of the shells is determined for 20 OH/IR stars in the galactic disk. It is measured on the base of a monitoring program with the Nancay radio telescope ongoing for more than 6 years. Recent eMERLIN and eVLA observations of several stars provided angular diameters.
The new distances for the sample will provide an improved estimate of the mass range from which these highly obscured stars descend from. I will discuss the new distances obtained and the uncertainties inherent in the method.