  In the recent literature there has been some doubt as to the reliability of
CO multi-transitional line observations as a mass-loss-rate estimator for AGB
stars. Mass-loss rates for 10 intermediate- to high-mass-loss-rate AGB stars
are derived using a detailed non-LTE, non-local radiative transfer code based
on the Monte-Carlo method to model the CO radio line intensities. The
circumstellar envelopes are assumed to be spherically symmetric and formed by
constant mass-loss rates. The energy balance is solved self-consistently and
the effects of dust on the radiation field and thermal balance are included. An
independent estimate of the mass-loss rate is also obtained from the
combination of dust radiative transfer modelling with a dynamical model of the
gas and dust particles. We find that the CO radio line intensities and shapes
are successfully reproduced for the majority of our objects assuming a constant
mass-loss rate. Moreover, the CO line intensities are only weakly dependent on
the adopted micro-turbulent velocity, in contrast to recent claims in the
literature. The two methods used in the present work to derive mass-loss-rates
are consistent within a factor of ~3 for intermediate- to high-mass-loss-rate
objects, indicating that this is a lower limit to the uncertainty in present
mass-loss-rate estimates. We find a tentative trend with chemistry. Mass-loss
rates from the dust/dynamical model are systematically higher than those from
the CO model for the carbon stars and vice versa for the M-type stars. This
could be ascribed to a discrepancy in the adopted CO/H_2-abundance ratio, but
we caution that the sample is small and systematic errors cannot be excluded.
