The analysis of indexed astronomical time Series – II. The O – C (observed – calculated) technique reconsidered

Abstract

Circumstances under which application of the O – C technique leads to spurious conclusions are described. For data in which measurements of many individual cycle lengths are available, use of the CUSUM method is a good alternative. The contentions of the paper are illustrated by application to observations of RY Sgr: in contrast to several previous authors, it is concluded that the mean pulsation period of the star is not changing.