Context : 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 .

Aims : The star CoRoT 102781750 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 .

Methods : 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 .

Results : 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 _ { \mathrm { rot } } = 8.8  hours ( f _ { 0 } = 2.735  c d ^ { -1 } ) mean rotation period .
The evolution of the active regions helped to derive a period change of dP / dt = 1.6 \cdot 10 ^ { -6 } ( 18Â s over the run ) and a differential rotation of \alpha = \Delta \Omega / \Omega = 0.0074 .
The 0 \aas@@fstack { m } 015 linear decrease and a local 0 \aas@@fstack { m } 005 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 f _ { 1 } = 7.172  c d ^ { -1 } pulsation connected to a 14 \aas@@fstack { d } 83 periodicity via an equidistant triplet structure .
The best explanation for our observation is a \beta Cep star with a corotating dust disk .

Conclusions :