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Context: The central star of the HR 8799 system is a gamma Doradus-typepulsator. The system harbours four planetary-mass companions detected by directimaging, and is a good solar system analogue. The masses of the companions arenot known accurately, because the estimation depends strongly on the age of thesystem, which is also not known with sufficient accuracy. Asteroseismic studiesof the star might help to better constrain the age of HR 8799. We organized anextensive photometric and multi-site spectroscopic observing campaign forstudying the pulsations of the central star.  Aims: The aim of the present study is to investigate the pulsation propertiesof HR 8799 in detail via the ultra-precise 47-d-long nearly continuousphotometry obtained with the MOST space telescope, and to find as manyindependent pulsation modes as possible, which is the prerequisite of anasteroseismic age determination.  Methods: We carried out Fourier analysis of the wide-band photometric timeseries.  Results: We find that resonance and sudden amplitude changes characterize thepulsation of HR 8799. The dominant frequency is always at f1 = 1.978 c/d. Manymultiples of one ninth of the dominant frequency appear in the Fourier spectrumof the MOST data: n/9 f1, where n={1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 13, 14, 17,18}. Our analysis also reveals that many of these peaks show strong amplitudedecrease and phase variations even on the 47-d time-scale. The dependenciesbetween the pulsation frequencies of HR 8799 make the planned subsequentasteroseismic analysis rather difficult. We point out some resemblance betweenthe light curve of HR 8799 and the modulated pulsation light curves of BlazhkoRR Lyrae stars.

MOST light-curve analysis of the gamma Dor pulsator HR 8799, showing resonances and amplitude variations