  Stellar evolution models are most uncertain for evolved massive stars.
Asteroseismology based on high-precision uninterrupted space photometry has
become a new way to test the outcome of stellar evolution theory and was
recently applied to a multitude of stars, but not yet to massive evolved
supergiants.Our aim is to detect, analyse and interpret the photospheric and
wind variability of the O9.5Iab star HD 188209 from Kepler space photometry and
long-term high-resolution spectroscopy. We used Kepler scattered-light
photometry obtained by the nominal mission during 1460d to deduce the
photometric variability of this O-type supergiant. In addition, we assembled
and analysed high-resolution high signal-to-noise spectroscopy taken with four
spectrographs during some 1800d to interpret the temporal spectroscopic
variability of the star. The variability of this blue supergiant derived from
the scattered-light space photometry is in full in agreement with the one found
in the ground-based spectroscopy. We find significant low-frequency variability
that is consistently detected in all spectral lines of HD 188209. The
photospheric variability propagates into the wind, where it has similar
frequencies but slightly higher amplitudes. The morphology of the frequency
spectra derived from the long-term photometry and spectroscopy points towards a
spectrum of travelling waves with frequency values in the range expected for an
evolved O-type star. Convectively-driven internal gravity waves excited in the
stellar interior offer the most plausible explanation of the detected
variability.
