2002A&A...387..969K

The effects of starspots on the O-C diagrams of eclipsing binaries are investigated, with a special emphasis on contact binaries. It is shown that starspots modulate the O-C differences in a fundamentally different way than real orbital period variations. Migrating starspots can only introduce high frequency, low amplitude disturbances (generally less than 0.01d), whose characteristics are numerically specified for typical contact binaries. It becomes evident that the observed systematic short timescale variations of the O-C diagrams of active contact binaries (even those like VW Cep) cannot be attributed to starspots. Furthermore, the basic property of non-linear modulation of O-C differences is emphasised. This non-linearity has serious consequences for the widespread traditional method of O-C diagram analysis. This traditional method is found inappropriate even on a fundamental level, and its mathematical and physical inconsistencies are reviewed.