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Astron. Astrophys. 355, 171-175 (2000)

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Orbital period studies of the two contact binaries TZ Bootis and Y Sextantis *

S. Qian 1,2,3 and Q. Liu 1,2,3

1 Yunnan Observatory, Chinese Academy of Sciences, P.O. Box 110, 650011 Kunming, P.R. China (qsb@netease.com)
2 National Astronomical Observatories, Chinese Academy of Science (NAOCAS)
3 United Laboratory of Optical Astronomy, Chinese Academy of Science (ULOAC)

Received 19 February 1999 / Accepted 9 December 1999


The physical properties of the two A-type contact binaries TZ Boo and Y Sex are nearly the same. In the present paper, many of their published times of light minima are collected and the changes in their orbital periods are analyzed. It is indicated that the orbital period of TZ Boo shows several alternating jumps while it undergoes a secular decrease of [FORMULA] days/year. Several random jumps superposed on a secular decrease ([FORMULA] days/year) are also found in the period of Y Sex. The secular decrease is usually interpreted as mass transfer from the more to the less massive components, or mass and angular momentum loss (AML) from the systems. According to the AML theory, on the contact stage, the orbital AML is mainly caused by the mass transfer from the less to the more massive component and the mass ratio decreasing and orbital period gradually increasing are the corresponding results. The extremely low mass ratio and orbital angular momentum of the two systems show that they are evolved via AML and the present secular decrease in the periods may suggest that the magnetic activity in the two systems are very strong. The relation between the changes of the orbital periods and the magnetic activity in the two systems are discussed. We think that the interplay between the variable AML and variable magnetic coupling can explain both the jumps and secular decrease in the orbital periods of the two systems.

Key words: stars: binaries: close – stars: individual: TZ Boo – stars: late-type – stars: magnetic fields – stars: individual: Y Sex

* Table 2 and Table 4 are only available in electronic form at the CDS via anonymous ftp to ftp://cdsarc.u-strabg.fr/pub/cats/J/A+A/355/171 ( or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/355/171

Send offprint requests to: Shengbang Qian

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© European Southern Observatory (ESO) 2000

Online publication: March 17, 2000