Non-radial pulsations in the O stars ξ Persei and λ Cephei

A new time-series analysis of profile changes in the photospheric He I λ4713 spectral line from data taken during 5 days in 1989 at the Calar Alto and Kitt Peak observatories has provided evidence for the presence of a non-radial prograde p-mode in the O 7.5 giant ξ Per (` = 3, P = 3.5) and probably two such modes in the O6 supergiant λ Cep (` = 3, P = 12.3 h and ` = 5, P = 6.6 h). The corotating pulsation periods are in both cases much shorter than the estimated stellar rotation period. Modeling the observed amplitude of the line-profile changes (assuming |m| = `) yields a velocity amplitude of approximately 5 km s−1 of the pulsation in ξ Per and 6 km s−1 in λ Cep. Any such a pulsation by itself is unlikely to be the cause of the well-known 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 candidate for the origin of wind variability.