Ratio between the square visibility in the continuum (average between 2.27 and 2.28 μm) and the square visibility in the CO (2−0) line vs. log(L/L_⊙) (top) and the variability amplitude (bottom) for a sample of RSGs in blue (Arroyo-Torres et al. 2013; Wittkowski et al. 2012; this work), AGB stars in red (Arroyo-Torres et al. 2014) and Mira variable stars in magenta (Wittkowski et al. 2011) for visibilities in the continuum between 0.2 and 0.4. The dotted black lines show the range of predictions by the best-fit PHOENIX model of these sources, the dashed orange the range of predictions by the best-fit RHD simulation. Both lines overlap.