The spectral variations of the O-type runaway supergiant HD 188209

We report spectral time series of the late O-type runaway supergiant HD 188 209, Radial velocity variations of photospheric absorption lines with a pos sible quasi-period of similar to 6.4 d have been detected in high-resolutio n echelle spectra. Night-to-night Variations in the position and strength o f the central emission reversal of the H alpha profile occurring over ill-d efined time-scales have been observed. The fundamental parameters of the st ar are derived using state-of-the-art plane-parallel and unified non-LTE mo del atmospheres, the latter including the mass-loss rate. The derived heliu m abundance is moderately enhanced with respect to solar, and the stellar m asses are lower than those predicted by the evolutionary models. The binary nature of this star is not suggested either from Hipparcos photometry or f rom radial velocity curves.