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

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 o f light minima are collected and the changes in their orbital periods are a nalyzed. It is indicated that the orbital period of TZ Boo shows several al ternating jumps while it undergoes a secular decrease of -11.8 x 10(-8) day s/year. Several random jumps superposed on a secular decrease (-5.5 x 10(-8 ) 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 com ponents, or mass and angular momentum loss (AML) from the systems. Accordin g 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 m ass ratio decreasing and orbital period gradually increasing are the corres ponding results. The extremely low mass ratio and orbital angular momentum of the two systems show that they are evolved via AML and the present secul ar decrease in the periods may suggest that the magnetic activity in the tw o systems are very strong. The relation between the changes of the orbital periods and the magnetic activity in the two systems are discussed. We thin k that the interplay between the variable AML and variable magnetic couplin g can explain both the jumps and secular decrease in the orbital periods of the two systems.