Radial velocities (RVs), with circles showing RVs derived from synthetic lines for two models, observation as crosses, and the grey line a running mean for the observations. The observed measurements are a compilation of RVs derived from FTS spectra for a large sample of different Mira stars, with the RVs converted to systematic velocities using the centre of mass radial velocity (CMRV) for each object (data from Lebzelter & Hinkle 2002).