Fourier analysis of the light curve of AC And from the HATNet database reveals the rich frequency structure of this object. Above 30 components are found down to the amplitude of 3 mmag. Several of these frequencies are not the linear combinations of
the three basic components. We detect period increase in all three components that may lend support to the Pop I classification of this variable.
During the past few years the Trend Filtering Algorithm (TFA) has become an important utility in filtering out time-dependent systematic effects in photometric databases for extrasolar planetary transit search. Here we present the extension of the me
thod to multiperiodic signals and show the high efficiency of the signal detection over the direct frequency analysis on the original database derived by todays standard methods (e.g., aperture photometry). We also consider the (iterative) signal reconstruction that involves the proper extraction of the systematics. The method is demonstrated on the database of fields observed by the HATNet project. A preliminary variability statistics suggests incidence rates between 4 and 10% with many (sub)mmag amplitude variables.
