Chromospheric activity and period variations of the contact binary EE Cet from TESS

Abstract

From the 100-day TESS observations, we comprehensively studied the variations of the light curve and orbital period for EE Cet. It is found that an 83.2-day oscillation occurs in the differences between two light maxima, which identifies the existence of chromospheric activity. Using the 2016-version Wilson–Devinney code, we obtained three sets of photometric solutions from TIC1432, TIC2449 and TIC2495. Results imply a dark spot immigrating from longitude 217.9(±2.2)∘ in BJD 2459449 into longitude 61.6(±0.7)∘ in BJD 2459495. From the eclipse timing residuals (i.e., observed minus computed), the orbital period of EE Cet may be undergoing a long-term period increase with a periodic variation. The 15.95-yr periodic oscillation with A=0.0038(1) d may result from the light-time effect due to the third body. Meanwhile, the orbital period increases at a rate of dP/dt=1.74(±0.02)×10−7dyr−1, which may result from the conserved mass transfer from the less massive component to the more massive one. With mass transferring, EE Cet will evolve into the broken-contact configuration, as predicted by the TRO theory.