  (Abridged) The spectral energy distributions of a well-defined sample of 54
RGB stars are constructed, and fitted with the dust radiative transfer model
DUSTY. The central stars are modeled by MARCS model atmospheres. In a first
step, the best-fit MARCS model is derived, determining the effective
temperature. In a second step, models with a finite dust optical depth are
fitted and it is determined whether the reduction in chi2 in such models with
one additional free parameter is statistically significant.
  23 stars are found to have a significant infrared excess, which is
interpreted as mass loss. The dust optical depths are translated into mass-loss
rates assuming a typical expansion velocity of 10 km/s and a dust-to-gas ratio
of 0.005.
  The mass-loss rates are compared to those derived for luminous stars in
globular clusters, by fitting both the infrared excess, as in the present
paper, and the chromospheric lines. There is excellent agreement between these
values and the mass-loss rates derived from the chromospheric activity. There
is a systematic difference with the literature mass-loss rates derived from
modeling the infrared excess, and this has been traced to technical details on
how the DUSTY radiative transfer model is run.
  If the present results are combined with those from modeling the
chromospheric emission lines, we obtain the fits Log Mdot = (1.0 +- 0.3) Log L
+ (-12.0 +- 0.9) and Log Mdot = (0.6 +- 0.2) Log (LR/M) + (-11.9 +- 0.9).
  The predictions of these mass-loss rate formula are tested against the RGB
mass loss determination in NGC 6791. Using a scaling factor of (8 +- 5), both
relations can fit this value. That the scaling factor is larger than unity
suggests that the expansion velocity and/or dust-to-gas ratio, or even the dust
opacities, are different from the values adopted.
