In a double or a multiple system, in which one of the components is a pulsating star, the light-time effect leads to apparent changes of the pulsation period. In some cases, one can obtain much more accurate values of the spectroscopic elements from the fit to the 0 — C diagram than from classical analysis of the radial-velocity data. In addition, in the analysis of the O — C diagram, photometric observations can be used. They are easy to obtain, and therefore are usually more numerous than the spectrographic ones.

Below we present the results of a study of three stellar systems, in which the light-time effect is observed. Primaries in all these systems are β Cephei-type pulsating variables.