FIRST MODERN PHOTOMETRIC INVESTIGATION OF THE PUZZLING W UMa TYPE CLOSE BINARY SYSTEM OF TZ BOOTIS

New CCD photometric BVRI observations of the puzzling W UMa type binary star, TZ Bootis, are presented from our observations in 2010. By using the updated version of the Wilson–Devinney code, the first modern photometric solution is deduced from new photometric observations and published spectroscopic data. This low mass ratio binary turns out to be a deep overcontact system with f = 52% of A-subtype. A spot model has been applied to fit the particular features of light curves. Based on our seven new light minimum times and all others compiled from the literature over 70 yr, we studied the orbital period from the O–C curve. It is found that a 31.2 yr cyclic variation exists with an amplitude of 0.033 days, overlaying a secular decrease at a rate of dP/dt = −2.1 × 10−8 days yr−1. The cyclic period change may indicate that TZ Boo is a triple or a quadruple system as confirmed from the published spectroscopic data. The long-term orbital period decrease is interpreted by mass transfer from the more to the less massive component and/or angular momentum loss by the magnetic breaking which would cause the overcontact degree to increase and finally the binary will evolve into a single rapidly rotating star.

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