An extensive photometric study of the Blazhko RR Lyrae star MW Lyr - I. Light-curve solution

We have obtained the most extensive and most accurate photometric data of a Blazhko variable MW Lyrae (MW Lyr) during the 2006-2007 observing seasons. The data within each 0.05 phase bin of the modulation period (Pm=f−1m) cover the entire light cycle of the primary pulsation period (P0=f−10), making possible a very rigorous and complete analysis. The modulation period is found to be 16.5462 d, which is about half of that was reported earlier from visual observations. Previously unknown features of the modulation have been detected. Besides the main modulation frequency fm, sidelobe modulation frequencies around the pulsation frequency and its harmonics appear at ±2fm, ±4fm and ±12.5fm separations as well. Residual signals in the pre-whitened light curve larger than the observational noise appear at the minimum-rising branch-maximum phase of the pulsation, which most probably arise from some stochastic/chaotic behaviour of the pulsation/modulation. The Fourier parameters of the mean light curve differ significantly from the averages of the Fourier parameters of the observed light curves in the different phases of the Blazhko cycle. Consequently, the mean light curve of MW Lyr never matches its actual light variation. The Φ21, Φ31 phase differences in different phases of the modulation show unexpected stability during the Blazhko cycle. A new phenomenological description of the light-curve variation is defined that separates the amplitude and phase (period) modulations utilizing the phase coherency of the lower order Fourier phase