Top panel: mass-loss rates and wind velocities of M-type AGB stars derived from observations of several CO lines (Olofsson et al. 2002; González Delgado et al. 2003), and the corresponding properties of models with the combination of log(n_d ∕n_H) = − 14.5 and u_p = 2.0, and with P < 800 days. The blue and red symbols indicate stars presumably pulsating in first overtone and fundamental mode, respectively. The dashed line is the linear fit to the observed values. Middle panel: mass-loss rate as a function of pulsation period for the same observational sample. Lower panel: difference between the mass-loss rates of the models and the expected values from the observational linear fit.