THE LIGHT CURVES OF PERIODIC BE STARS IN NGC-3766

In this paper we present results of an intensive photometric campaign on four periodic Be stars (lambda-Eri variables) in the southern open cluster NGC 3766. Light curves covering the years 1985-90 are presented. We find 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. This represents the largest amount of data for any lambda-Eri star and we are able to state conclusively that within the observational limits there is no evidence at all for coherent multiperiodicity in any of the four stars. Incoherent variability (flickering) is often present. We find no obvious correlation between the amplitudes or shapes of the light curves and the H-alpha line profiles. We discuss these findings in the light of the non-radial pulsation and starspot hypotheses and show that they are observationally indistinguishable. Both are incapable of accounting for the observed radial velocity variations. We discuss a model in which matter is trapped in a magnetic field and forced into corotation with the star. This model explains the observed periods, the double-wave nature of most lambda-Eri stars, the phase locking, and probably the line profile variations as well.