2020ApJ...901..169L

We present new photometric data and LAMOST spectra for the W UMa binaries UV Lyn, V781 Tau, NSVS 4484038, and 2MASS J15471055+5302107. The orbital and starspot parameters are obtained using the Wilson-Devinney program. Comparing the starspot parameters at different times, there are magnetic activities in these four binaries. The orbital period of UV Lyn is increasing at a rate of dP/dt = +8.9(5) x 10^–8 days yr^–1, which maybe due to mass transfer from the less massive component to the more massive component (dM₁/dt = -6.4 x 10^–8 M_☉ yr^–1). The period variation of 2MASS J15471055+5302107 is also increasing at a rate of 6.0(4) x 10^–7 days yr^–1, which can be explained by mass transfer from the less massive component to the more massive component (dM₁/dt = -2.8 x 10^–7 M_☉ yr^–1). The period variation of V781 Tau presents the downward parabola superimposed the cyclic oscillation. The period of V781 Tau is decreasing (dP/dt = -3.2(4) x 10^–8 days yr^–1), which can be explained by mass transfer from the more massive component to the less massive component (dM₂/dt = -2.2 x 10^–8 M_☉ yr^–1). The cyclic oscillation may be due to the magnetic activity with a period of 30.8(5) yr rather than a third body. The period variation of NSVS 4484038 also shows the cyclic oscillation, which could be explained by the magnetic activity with 10.8(1) yr or a black hole candidate. Interestingly, there is a depth variation between the light minimum times of NSVS 4484038, which may also be caused by stellar magnetic activity.