In the exoplanet (and, before that, eclipsing-binary) communities, transit-timing variations are described in terms of a quantity called O−C (pronounced “oh minus sea”), which is the difference between the observed transit time and the “computed” transit time. Right now, Abby Shaum (CUNY) and I are using this terminology in our manuscript about phase variations in coherent pulsators with companions, at the behest of Keaton Bell (CUNY). Okay fine! But O−C has this terrible property, which is that the C part depends on the period or frequency you assume. You can completely change the appearance or morphology of an O−C plot just by slightly tweaking the period. And there is no true period of course! There is just whatever estimates you can make. Which are, in turn, affected by what you use to model the O−C. So it is absolutely awful in every way. Not a stable observable, people! Not even identifiable.
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