An explanation of the change in the orbital period of the contact binary V781 Tauri

New times of minimum light determined from the observations of the W UMa type contact binary V781 Tau are presented. According to these times of minima and those collected from the references, the change in the orbital period of the system is analyzed. The result reveals that the orbital period of V781 Tau decreased continually from 1949 to 1998 and the change rate in the period isbp/p=—5.0 x 10-11.The decrease in the period of the binary is explained by a model of the contracting secondary component of the system. The shrinking velocity of 6.77x 10-5crns-1as responsible for the change in the orbital period of the system is obtained, which is in agreement with the results shown in the study of the thermal relaxation oscillation of contact binaries by Wang (1994).