I-band amplitudes of our sample stars. Symbols are as in Fig. 1. The amplitudes are estimated peak-to-peak variations. The values for LMC stars are taken from Hughes & Wood (1990). For the SMC stars, we estimated amplitudes using OGLE light curves (available through the Vizier service at CDS). For the MW, we estimated amplitudes based on K-band amplitudes by Whitelock et al. (2006); a value of 0.5 was assigned when no data were available. We note that for two MW stars (filled squares) large-amplitude luminosity dips are known to occur occasionally in addition to small-amplitude variations (the R CrB phenomenon).