2002PASP..114..650Q

O-C curves of two EA-type eclipsing binaries, TY Peg and X Tri, are formed and studied in detail based on all available times of light minimum. For TY Peg, it was discovered that the general trend of the O-C curve showed a downward parabolic variation, which indicates a secular period decrease with rate at dP/dt=-1.16x10^–6 days.yr^–1. When the secular decrease is removed from the O-C curve, the residuals can be described by several linear ephemerides, suggesting several sudden period jumps. For the other one, X Tri, it is found that the general O-C trend also indicates a secular period decrease (dP/dt=-1.42x10^–7 days.yr^–1), and several irregular period jumps are found to superpose on the long-time period decrease. This kind of period change, irregular sudden jumps superposed on secular decrease, is typical for many other Algol-type binary systems such as TU Her, UU And, Z Per, Y Psc, BO Mon, FH Ori, RT Per, and TX UMa. The period changes of these systems may be caused by orbital angular momentum loss via variable magnetic coupling and gravity coupling between the components.