2MASS J06452845+0023023 , the SIMBAD biblio

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.