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Description
This paper presents JHKL'M photometry of a new sample of OH/IR stars. These data are merged with similar observations collected in the literature in order to constitute a data base of near-infrared photometry for 400 OH/IR stars. After removing from this sample the objects that are unlikely to be AGB stars, we obtain a sequence of average OH/IR stars with increasing values of the colour index K-L'. A simple model of a star surrounded by a circumstellar shell is constructed in order to derive basic parameters such as luminosity, radius, stellar temperature, optical depth of the dust shell and mass loss rate. These parameters are found to vary smoothly along the sequence of OH/IR stars, and the K-L' colour is shown to describe almost completely the basic physical parameters of an OH/IR star. The metallicity affects the OH-peak separation, but does not play an important role in the definition of the other properties.
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