The period variations of three Algol-type binaries, TT Del, BO Mon, and Y Psc, are studied based on the analysis of the O-C observations. The orbital period of TT Del is discovered to be decreasing at a rate dP/dt = -2.20 × 10-6 days yr-1, while the orbital periods of BO Mon and Y Psc vary in some complex ways. For BO Mon, three abrupt changes in its orbital period are found while it undergoes secular decrease at a rate dP/dt = -2.36 × 10-6 days yr-1. For the remaining one, Y Psc, two jumps in the orbital period are found to superpose on a rapid period decrease (dP/dt = -4.90 × 10-6 days yr-1). As in the case of TU Her (Paper I), the rapid decrease in the orbital periods of these systems, combined with their semidetached configurations, suggests that they may be undergoing rapid mass and angular momentum loss (AML) due to magnetic braking. With the present decrease rate, after (7–9) × 105 yr they will evolve into contact binaries. The abrupt changes that are superposed on the rapid period decrease may be caused by the interplay between the variable AML and variable magnetic coupling.
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