2011A&A...531A.135P

The CoRoT space mission (COnvection, ROtation and planetary Transits) launched in December 2006, aims at finding transiting exoplanets and investigating stellar oscillation in adjacent stellar fields, called exo- and seismofields, respectively. Besides the seismofields, CoRoT has a strong potential for seismological research on the exofields. Up to now, only a limited number of RR Lyrae stars have been classified among the CoRoT targets. Knowing the astrophysical importance of the RR Lyrae stars, we attempted to get useful information even from a contaminated light curve of a possible RR Lyrae pulsator. The star CoRoT102781750 reveals a puzzle, showing a very complex and altering variation in different ``CoRoT colours''. We established without doubt that more than a single star was situated within the CoRoT mask. Using a search for periodicity as a tool, our aim is to disentangle the composite light curve and identify the type of sources behind the variability. Both flux and magnitude light curves were used. Conversion was applied after a jump- and trend-filtering algorithm. We applied different types of period-finding techniques including MuFrAn and Period04. The amplitude and phase peculiarities obtained from the independent analysis of CoRoT r, g, and b colours and ground-based follow-up photometric observations ruled out the possibility of either a background monoperiodic or a Blazhko type RR Lyrae star being in the mask. The main target, an active star, shows at least two spotted areas that reveal a P_rot=8.8h (f₀=2.735c/d) mean rotation period. The evolution of the active regions helped to derive a period change of dP/dt=1.6x10^–6 (18s over the run) and a differential rotation of α=ΔΩ/Ω=0.0074. The 0.015mag linear decrease and a local 0.005mag increase in the dominant period's amplitude are interpreted as a decay of the old spotted region and an appearance of a new one, respectively. A star that is detected only in the CoRoT b domain shows a f₁=7.172c/d pulsation connected to a 14.83d periodicity via an equidistant triplet structure. The best explanation for our observation is a β Cep star with a corotating dust disk.