We studied an RS Canum Venacorum type star BD Ceti. It is a chromospherically active binary (CAB). The primary component is a giant star, but shows solar type activity. With the orbital period of 35.1 days, it belongs into the long period group of RS CVn stars. The tidal interaction between the binary components has accelerated the rotation of the active component so that orbital and rotational period are almost the same. This fast rotation maintains magnetic and chromospheric activity.
We applied the Continuous Period Search (CPS) method to 20 years automated V-band photometry. Modelling gave us mean magnitudes, amplitudes, periods, and primary and secondary minima of the light curve, as well as their error estimates. We studied both long and short term variations of the star.
We found a possible 17.9 years activity cycle in the minimum starspot coverage. Because this cycle covers 90 % of the time span of our data, we could not confirm if this is a real phenomenon.
From the period changes, we got the differential rotation coeffient |k| > 0.14, if the activity is present from the equator to the poles. We compared our result with latitudinal spot activity of the Sun and another RS CVn star Sigma Geminorum. BD Ceti has a higher differential rotation rate. We compared our result also to other published relations for estatimating differential rotation. We found that our results agree with the predictions by Collier Cameron (2007) and Küker & Rüdiger (2011) and Reinhold et al. (2013). Our relatively high differential rotation estimate is consistent to published results where slower rotating stars have higher differential rotation.
We found two active longitudes located almost at the opposite sides of the star. The primary active longitude existed through entire 20 years observing period. The second active longituge existed only occasionally. No evidence of occasional activity switch, so called ‘flip-flops’, between these longitudes was not found. We got a period of 34.6 ± 0.1 days for those longitudes, which is close to orbital period of the binary system. This active longitude structure may be an evidence of azimuthal magnetic dynamo rotating slightly faster than the star.
