The nature of OH/IR stars. I. Infrared Mira variables.

Seventeen infrared counterparts of OH/IR sources were measured in the wavelength range 1 to 30 microns between 1976 and 1980. For 15 of these OH/IR stars, periods between 500 and 1800 d and amplitudes up to 4 mag were determined at K. By comparison with known variable stars showing the same characteristic OH maser emission, these OH/IR stars must be considered as an extension of the optically known Mira stars toward greater masses and longer periods. The long period pulsation of these 'infrared Mira stars' is connected with enhanced mass loss, which causes the formation of optically thick circumstellar shells. These dust shells absorb essentially all of the optical emission of the central star. The classification of the OH/IR stars as Mira variables implies that intermediate mass stars (2-10 solar masses) are also going through a phase of long period pulsation during their evolution on the asymptotic giant branch.