Orbital period change of SW Sex star TT Tri

SW Sex-type stars are the dominate population of cataclysmic variables (CVs) just above the period gap and are vital for understanding of CV evolution. We have monitored TT Tri for nearly 15 yr, and more observations yield a more reliable estimation for its orbital period variation. In this work, we present a detailed analysis about its orbital period variation. TT Tri experiences a rapid period decay with a decay rate of -5.67× 10⁻¹¹\, s/s plus a period oscillation at about 9.6 yr. The oscillation could be caused by the magnetic activities of the low-mass star or/and a circumbinary body, while the rapid period decrease cannot be accounted for by the magnetic braking or even including an intensive disc wind. We find that mass overflow though the L3 point can take the angular momentum away effectively and that this mass overflow could account for the rapid period decay observed in TT Tri. We propose that this kind of angular momentum loss may play a crucial role in CV evolution, and further research is in progress.