Right: synthetic  [3.6]  −  [4.5]  of RSG stars versus mass-loss rates. Magnitudes are obtained from ISO-SWS spectra (Sloan et al. 2003), and mass-loss rates are given by Verhoelst et al. (2009). Starred symbols are RSG stars with periodic pulsations (SR), while diamonds are irregular RSG stars. Left: synthetic  [3.6]  −  [8.0]  of RSG stars modeled by Verhoelst et al. (2009) versus their inferred mass-loss rates. Starred symbols are RSG stars with periodic pulsations (SR), while diamonds are irregular RSG stars.