The light-time effect as the cause of period changes in beta Cephei stars. II. Sigma Scorpii.

The O-C diagram for the main pulsation period of the Beta Cephei-type star Sigma Scorpii indicates that the period increased in the first half of the century, decreased after 1960, and again increased about 1984. We show that these changes of the observed period can be explained by a superposition of two effects: an evolutionary increase of the intrinsic pulsation period and a variation due to the light-time effect in a binary system. The star responsible for the latter effect is the system's component detected by means of the lunar occultations and speckle interferometry. A companion at a similar separation was recently found to cause the observed period changes in Beta Cep, the prototype of this class of variables. In this way, Sigma Sco becomes the second beta Cephei-type star in which the light-time effect in a binary system contributes to the observed changes of the pulsation period. The rate of the secular period change found for Sigma Sco locates this star in the shell-hydrogen burning phase. Since a similar rate of period change is observed in BW Vul, we note the possible connection between evolutionary status and the pulsation amplitudes in Beta Cephei-type stars.