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) x 10(-8) days yr(-1), which maybe due to mass transfer from the less massive component to the more massive component (dM(1)/dt = -6.4 x 10(-8) M-circle dot 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 (dM(1)/dt = -2.8 x 10(-7) M-circle dot 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 (dM(2)/dt = -2.2 x 10(-8) M-circle dot 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.