We have determined mass loss rates and gas expansion velocities for a sample of 69 M-type irregular ( IRV ; 22 objects ) and semiregular ( SRV ; 47 objects ) AGB-variables using a radiative transfer code to model their circumstellar CO radio line emission .
We believe that this sample is representative for the mass losing stars of this type .
The ( molecular hydrogen ) mass loss rate distribution has a median value of 2.0 \times 10 ^ { -7 } M _ { \odot } yr ^ { -1 } , and a minimum of 2.0 \times 10 ^ { -8 } M _ { \odot } yr ^ { -1 } and a maximum of 8 \times 10 ^ { -7 } M _ { \odot } yr ^ { -1 } .
M-type IRVs and SRVs with a mass loss rate in excess of 5 \times 10 ^ { -7 } M _ { \odot } yr ^ { -1 } must be very rare , and among these mass losing stars the number of sources with mass loss rates below a few 10 ^ { -8 } M _ { \odot } yr ^ { -1 } must be small .
We find no significant difference between the IRVs and the SRVs in terms of their mass loss characteristics .
Among the SRVs the mass loss rate shows no dependence on the period .
Likewise the mass loss rate shows no correlation with the stellar temperature .

The gas expansion velocity distribution has a median of 7.0 km s ^ { -1 } , and a minimum of 2.2 km s ^ { -1 } and a maximum of 14.4 km s ^ { -1 } .
No doubt , these objects sample the low gas expansion velocity end of AGB winds .
The fraction of objects with low gas expansion velocities is very high , about 30 % have velocities lower than 5 km s ^ { -1 } , and there are objects with velocities lower than 3 km s ^ { -1 } : V584~Aql , T~Ari , BI~Car , RX~Lac , and L $ ^2 $ ~Pup .
The mass loss rate and the gas expansion velocity correlate well , a result in line with theoretical predictions for an optically thin , dust-driven wind .

In general , the model produces line profiles which acceptably fit the observed ones .
An exceptional case is R~Dor , where the high-quality , observed line profiles are essentially flat-topped , while the model ones are sharply double-peaked .

The sample contains four sources with distinctly double-component CO line profiles , i.e. , a narrow feature centered on a broader feature : EP~Aqr , RV~Boo , X~Her , and SV~Psc .
We have modelled the two components separately for each star and derive mass loss rates and gas expansion velocities .

We have compared the results of this M-star sample with a similar C-star sample analysed in the same way .
The mass loss rate characteristics are very similar for the two samples .
On the contrary , the gas expansion velocity distributions are clearly different .
In particular , the number of low-velocity sources is much higher in the M-star sample .
We found no example of the sharply double-peaked CO line profile , which is evidence of a large , detached CO-shell , among the M-stars .
About 10 % of the C-stars show this phenomenon .