Standardization and calibration in the variability time domain
This talk presents a discussion of new and archival photometric measurements of several stars with complex spectral features, in particular the most massive stars (LBV and B[e] single and binary supergiants, WR stars, etc.). The data outline significant systematic differences between all involved systems of photometric measurement. The conclusion of this study is that objects that slowly change magnitude and color are most difficult objects to standardize, especially when data covering years or decades are applied. Sky surveys, with their short lifetimes, are particularly prone to systematic effects.