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1 Introduction

One of many interesting problems in the study of W UMa contact binaries is to explain the change in the orbital period of the systems. The variety of the period changes of W UMa binaries makes it difficult to understand their real physical mechanism. However, it is clear that the change in the period of a W UMa contact binary should result from their unstable structure and evolution. According to Lucy (1976), Flannery (1976) and Robertson & Eggleton (1977), a zero-age W UMa binary is obliged to undergo periodic thermal relaxation oscillation about a state of marginal contact. It can be expected that some observable changes in the orbital period of the systems should arise because of such oscillations. Unfortunately, the time scale of 107 years of the oscillations is too long to be observed completely for the short astronomical history, but it is noticed that the results obtained by Wang (1994) could provide a way to do something for this test. Wang's study suggests that the two subtypes of W UMa type binaries are in two different thermal relaxation oscillation states: the secondary components of W-subtypes of W UMa type binaries are shrinking whereas the ones of A-subtypes are swelling. Therefore, the orbital period of the W-subtypes should be continually decreasing whereas that of the A-subtypes should be continually increasing. For acquiring the observational evidence to test the results of Wang's study (1994) the authors are interested in the observation and analysis of the change in the period of a W UMa type binary. The W UMa type binary V 781 Tau is the first of our program stars.

The variability of V 781 Tau was discovered by Harris (1979), who classed it as a new W UMa type binary with the orbital period of $0\hbox{$.\!\!^{\rm d}$ }3994$. Berthold (1981) derived 14 times of minimum light of the system from 232 photographic plates taken from 1958 to 1977 and showed an orbital period with a sudden increase in 1.5 seconds in 1971. After two years he presented a revision of his previous results (Berthold 1983). The photoelectric times of minimum light of V 781 Tau were published by Diethelm (1981a, 1981b), Pohl (1987) and Keskin (1989). Cereda et al. (1988) made extensive photoelectric observations in B and V bands and published very good light curves. They found a photometric solution of their light curves and obtained rough parameters of the binary. The observations of the radial velocity and absolute dimensions were published by Lu (1993), who re-analyzed the Cereda et al.'s light curves by using the Wilson-Devinney code, with the results revealing that V 781 Tau is a W-subtype W UMa binary and the primary component resembles a main-sequence star and the secondary is very close to the ZAMS in the HR diagram and in the mass-radius diagram.


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