The tendency for the period
changes in eclipsing binaries to appear sinusoidal led astronomers in the first
half of this centruy to suggest apsidal motion or orbital motion around a third
body as explanations. Clear-cut examples of both of these phenomena have been
found; but as more and more time of minimum are observed and the base line in
time gets longer, it has become clear that in the large majority of cases the
period variations are not really sinusoidal or even periodic.