  12CO and 13CO lines, as well as a mm-wave continuum, have been observed for a
sample 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 to
determine pulsational variability. Radiative transfer models have been used to
interpret the data. All stars in the sample were detected in at least one CO
line, and 8 objects were detected in 324 GHz continuum. Based on luminosity
criteria, the sample is divided into 17 objects that most likely lie within the
inner 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 these
OH/IR stars descend from solar-type stars. The objects in this sub-sample are
further divided into two classes based on their SED characteristics: 11 objects
have SEDs that are well matched by models invoking dust envelopes extending
from a few stellar radii and outwards, while 6 objects are better modelled as
having detached dust envelopes with inner radii in the range 200-600 au and
warmer central stars. The former objects have periodic variability, while the
latter objects are predominantly non-periodic. The median gas-mass-loss rate,
gas terminal expansion velocity, gas-to-dust mass ratio, and circumstellar
12CO/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 markedly
lower gas-to-dust ratios), and 5, respectively, for the inner-Galactic-Bulge
objects. The inner-Galactic-Bulge OH/IR stars studied here constitute an
excellent sample of equidistant objects for the purpose of understanding the
evolution of the mass-loss-rate characteristics at the tip of the AGB.
