MOST photometry of the RRd Lyrae variable AQ Leo: two radial modes, 32 combination frequencies and beyond

Highly precise and nearly uninterrupted optical photometry of the RRLyrae star AQLeo was obtained with the MOST (Microvariability and Oscillations of Stars) satellite over 34.4d in 2005 February-March. AQLeo was the first known double-mode RRLyrae pulsator (RRd star). Three decades after its discovery, MOST observations have revealed that AQLeo oscillates with at least 42 frequencies, of which 32 are linear combinations (up to the sixth order) of the radial fundamental mode and its first overtone. Evidence for period changes of these modes is found in the data. The other intrinsic frequencies may represent an additional non-radial pulsation mode and its harmonics (plus linear combinations) which warrant theoretical modelling. The unprecedented number of frequencies detected with amplitudes down to mmag precision also presents an opportunity to test non-linear theories of mode growth and saturation in RRd stars.