Observed and synthetic photometric variations of M-type AGB stars. Top panels: photometric variations for a sample of observed targets, derived from sine fits of light-curves; near-infrared data by Whitelock et al. (2000) has been combined with visual data by Eggen (1975) and Mendoza (1967); see Bladh et al. (2013) for details. Middle panels: photometric variations of the DARWIN models listed in Table 1 with colors calculated from sine fits of the light-curves, same as for the observational data in the top panels; the lines are color-coded by stellar parameters (l50t28 = A, l70t27 = B) and pulsation amplitude (u3/u4). Bottom panels: the same model results, but color-coded by seed particle abundance (nd316 stands for 3 × 10^-16, nd115 for 1 × 10^-15, etc.). The right panels show the same content as the left panels, zoomed in and centered on the color loops. The single-epoch photometric data shown in the background of all panels represents Galactic Bulge miras (Groenewegen & Blommaert 2005, gray plus signs) and field M-type long-period variables (Mendoza 1967, black crosses).