Circumstellar CO around bright oxygen-rich semi-regulars.

We report high signal to noise millimeter wave observations of 11 nearby Semi-Regular variables, most with periods between 89 and 160 days, in the transitions ^12^CO (J=2-1) and (J=1-0) and ^13^CO (J=2-1). The line profiles have been fitted by a model including molecular excitation and radiative transfer and the derived envelope properties (expansion velocity, mass loss rate, outer radius, kinetic temperature distribution) are compared to the predictions of theoretical models. The mass loss properties of this sample of Semi-Regulars appear to be very similar with those of longer period (300<=P<400 days) Miras, observed with the same telescope. This result is consistent with the view that the Semi-Regulars are overtone pulsators while the Miras are in the fundamental pulsational mode. The pulsational mode does not strongly affect the mass loss from the star.