Aslan, Zeki2019-06-282019-06-28198110.1093/mnras/195.1.31http://hdl.handle.net/20.500.12575/66904Optical velocities of semi-regular (SR) and irregular (Lb) variables are analysed for a K-term. There is evidence for a dependence upon stellar period. Absorption lines in shorter period non-emission SR variables are blue-shifted relative to the centre-of-mass velocity by about 6±3 km s−1. Emission-line SR variables give a non-negative absorption K-term and Lb variables give no K-terms other than zero. Comparison is made with the K-terms implied by the OH velocity pattern in long-period variables. Dispersion ratios are also calculated. They are σv/σu=0.69±0.16 for non-emission, and 0.74±0.20 for emission variables (from absorption lines). Kinematics suggest that the sequence Lb–SR (non-emission)–SR (emission)–Mira is a sequence of increasing age. It is suggested that this sequence may also be the sequence indicating the direction of evolution.enArticle19513134