Volume 48 number 1 (2020)
(Abstract only) The O–C (“observed minus computed”) diagram is a simple concept: fit the observational data with a model and examine the quality of the fit. That is, for all the observed datapoints, such as the times of eclipses of an eclipsing binary star, or times of maximum of a pulsator, subtract the times the model would predict, and then plot those “residuals” of the model fit. This allows one to magnify the residuals and study them. Patterns in the residuals may explain how the system has changed, how the model should be adjusted, or whether a different form of model should be used. This discussion presents the concept, describes the mathematical procedure and techniques to calculate this term, and provides specific examples to explain what certain patterns mean in the study of eclipsing binary systems.