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12CO and 13CO lines, as well as a mm-wave continuum, have been observed for asample of 22 OH/IR stars in directions within 2 degrees of the Galactic Centre.Photometry data have been gathered from the literature to construct SEDs and todetermine pulsational variability. Radiative transfer models have been used tointerpret the data. All stars in the sample were detected in at least one COline, and 8 objects were detected in 324 GHz continuum. Based on luminositycriteria, the sample is divided into 17 objects that most likely lie within theinner Galactic Bulge, and 5 objects that are most likely foreground objects.The median luminosity of the inner-Galactic-Bulge objects, 5600 Lsun,corresponds to an initial mass in the range 1.2-1.6 Msun, indicating that theseOH/IR stars descend from solar-type stars. The objects in this sub-sample arefurther divided into two classes based on their SED characteristics: 11 objectshave SEDs that are well matched by models invoking dust envelopes extendingfrom a few stellar radii and outwards, while 6 objects are better modelled ashaving detached dust envelopes with inner radii in the range 200-600 au andwarmer central stars. The former objects have periodic variability, while thelatter objects are predominantly non-periodic. The median gas-mass-loss rate,gas terminal expansion velocity, gas-to-dust mass ratio, and circumstellar12CO/13CO abundance ratio have been estimated to be 2x10{-5} Msun/yr, 18 km/s,200 (excluding the sources with detached dust envelopes, which show markedlylower gas-to-dust ratios), and 5, respectively, for the inner-Galactic-Bulgeobjects. The inner-Galactic-Bulge OH/IR stars studied here constitute anexcellent sample of equidistant objects for the purpose of understanding theevolution of the mass-loss-rate characteristics at the tip of the AGB.

CO line observations of OH/IR stars in the inner Galactic Bulge: Characteristics of stars at the tip of the AGB