Non-radial pulsations in the O stars xi Persei and lambda Cephei

A new time-series analysis of profile changes in the photospheric He I lamb da 4713 spectral line from data taken during 5 days in 1989 at the Calar Al to and Kitt Peak observatories has provided evidence for the presence of a non-radial prograde p-mode in the O 7.5 giant xi Per (l = 3, P = 3.5) and p robably two such modes in the O6 supergiant lambda Cep (l = 3, P = 12.3 h a nd l = 5, P = 6.6 h). The corotating pulsation periods are in both cases mu ch shorter than the estimated stellar rotation period. Modeling the observe d amplitude of the line-profile changes (assuming \m\ = l) yields a velocit y amplitude of approximately 5 km s(-1) of the pulsation in xi Per and 6 km s(-1) in lambda Cep. Any such a pulsation by itself is unlikely to be the cause of the well-know n cyclical wind variability in these stars because the pulsation period is too short, but the cumulative action of multiple modes could cause such an effect. Weak magnetic fields anchored at the surface remain the strongest c andidate for the origin of wind variability.