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Stellar evolution models are most uncertain for evolved massive stars.Asteroseismology based on high-precision uninterrupted space photometry hasbecome a new way to test the outcome of stellar evolution theory and wasrecently applied to a multitude of stars, but not yet to massive evolvedsupergiants.Our aim is to detect, analyse and interpret the photospheric andwind variability of the O9.5Iab star HD 188209 from Kepler space photometry andlong-term high-resolution spectroscopy. We used Kepler scattered-lightphotometry obtained by the nominal mission during 1460d to deduce thephotometric variability of this O-type supergiant. In addition, we assembledand analysed high-resolution high signal-to-noise spectroscopy taken with fourspectrographs during some 1800d to interpret the temporal spectroscopicvariability of the star. The variability of this blue supergiant derived fromthe scattered-light space photometry is in full in agreement with the one foundin the ground-based spectroscopy. We find significant low-frequency variabilitythat is consistently detected in all spectral lines of HD 188209. Thephotospheric variability propagates into the wind, where it has similarfrequencies but slightly higher amplitudes. The morphology of the frequencyspectra derived from the long-term photometry and spectroscopy points towards aspectrum of travelling waves with frequency values in the range expected for anevolved O-type star. Convectively-driven internal gravity waves excited in thestellar interior offer the most plausible explanation of the detectedvariability.

Kepler sheds new and unprecedented light on the variability of a blue supergiant: gravity waves in the O9.5Iab star HD 188209