Magnetic Activity and Period Variation Studies of the Four W Uma-type Eclipsing Binaries: UV Lyn, V781 Tau, NSVS 4484038, and 2MASS J15471055+5302107

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) × 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 × 10−8 M yr−1). The period variation of 2MASS J15471055+5302107 is also increasing at a rate of 6.0(4) × 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 × 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) × 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 × 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.