Variable stars of small amplitude. III. Semi-period of variation for seven B2 to G0 supergiant stars.

The photometric observations of seven B2 to G0 Ia-type supergiants in the Geneva system are analyzed. All stars are found to be semi-periodic variables. The peak-to-peak amplitude of variation is about 1.5 times larger for the four B2 to A2 supergiants (about 0.06 min in msubv) than for the three A5 to G0 supergiants. A method of auto-correlation is used to determine the semi-period of variation P, and the accuracy of these determinations is in most cases very good. The P-value increases from 15-20 days for the B supergiants to more than 80 days for the G0 Ia star. This fact confirms the dependence on the spectral type of the semi-period-luminosity relation for the supergiant stars.