The light curves of periodic Be stars in NGC 3766

Results of an intensive photometric campaign on four period Be stars in the southern open cluster NGC 3766 are presented as well as light curves covering the years 1985-1990. It is found that, in spite of dramatic changes in the shapes and amplitudes of the light curves, phase locking is maintained and the period is unchanged over the five-year interval. It is concluded that within observational limits there is no evidence for coherent multiperiodicity in any of the four stars. No obvious correlation is found between the amplitudes or shapes of the light curves and the H-alpha line profiles. These findings are discussed in the light of the nonradial pulsation and starspot hypotheses, and they are shown to be observationally indistinguishable. A model in which matter is trapped in a magnetic field and forced into corotation with the star is discussed.