Long-term Activity in Photospheres of Low-mass Stars with Strong Magnetic Fields

The behavior of the average annual luminosity of K–M dwarfs OU Gem, EQ Vir, V1005 Ori and AU Mic was studied at time intervals of several decades. The main sources of photometric data for 1989–2019 were the Hipparcos, ASAS, KWS databases. An analysis of long-term series showed that the average annual brightness of all stars varies cyclical. The durations of possible cycles for OU Gem, V1005 Ori and AU Mic are 40–42 years, for EQ Vir—16.6 years, and amplitudes of cycles are of 0.09–0.2 m . The selected stars belong to the group of red dwarfs for which the average surface magnetic field 〈 B 〉 exceeds of several kilogauss. We examined the type of the relationship between the parameters of the cycle, its duration and amplitude, for 9 stars with 〈 B 〉 < 4 kG. A tendency to increasing of the amplitude and duration of the cycle when the value of 〈 B 〉 decrease has been noted. It may be suggested that with an increasing of the surface magnetic field, it becomes more uniform and the level of activity changes in this case in less degrees than on stars with strong local fields concentrated in large spots.