RACE-OC (Rotation and ACtivity Evolution in Open Clusters) is a project aimed at characterizing the rotational and magnetic activity properties of the late-type members of open clusters, stellar associations, and moving groups of different ages. The evolution in time of rotation and activity at different masses shed light on the evolution of the stellar internal structure, on magneto-hydro-dynamic processes operating in the stellar interior, and on the coupling/decoupling mechanisms between radiative core and external convective envelope. As part of this project, in the present paper we present the results of an investigation of a likely member of the AB Doradus association, the M-type star 2MASS J15594729+4403595. In the present study we aimed to reveal the real nature of our target, which turned out to be a hierarchical triple system, to derive the stellar rotation period and surface differential rotation, and to characterize its photospheric magnetic activity. We have collected RV and photometric time series, complemented with archive data, to determine the orbital parameters and the rotation period and have used the spot modeling technique to explore what causes its photometric variability. We found 2MASS J15594729+4403595 to be a hierarchical triple system consisting of a dwarf SB1 M2 and a companion M8. The M2 star has a rotation period P=0.37d, making it the fastest among M-type members of AB Dor. The most relevant result is the detection of a periodic variation of the spotted area on opposite stellar hemispheres which resembles a sort of Rossby waves or Rieger-like cycles on an extremely short time scale. Another interesting result is the occurrence of a highly significant photometric periodicity P=0.443d, which may be related to the stellar rotation either in terms of a Rossby wave or of surface differential rotation. 2MASS J15594729+4403595 may be the proto-type of a new class of extremely fast rotating stars exibiting short Rieger-like cycles. We will further explore what may drive these short-duration cycles as well as will search for other similar stars to allow a statistical analysis.

Cone search capability for table J/A+A/691/A117/list (List of fits spectra)