TY Pup: a low-mass-ratio and deep contact binary as a progenitor candidate of luminous red novae

TY Pup is a well-known bright eclipsing binary in southern hemisphere with an orbital period of 0.8192 days. New light curves in B, V, (RI)_C bands were obtained with the 0.61-m reflector robotic telescope (PROMPT-8) at CTIO in Chile from January to February 2015 and from March to April 2017. By analyzing those photometric data with the W-D method, it is found that TY Pup is a low-mass-ratio (q ∼ 0.184) and deep contact binary with a high fill-out factor (84.3 %). An investigation of all available times of minimum light including three new ones obtained with the 60-cm and the 1.0-m telescopes at Yunnan Observatories in China indicates that the period change of TY Pup is complex. An upward parabolic variation in the O-C diagram is detected to be superimposed on a cyclic oscillation. The upward parabolic change reveals a long-term continuous increase in the orbital period at a rate of dP/dt = 5.57(± 0.08)×10^-8 d yr^-1. The period increase can be explained by mass transfer from the less massive component (M_2 ∼ 0.3 M_⊙) to the more massive one (M_1 ∼ 1.65 M_⊙) and the mass ratio of the binary system will become more smaller. In this way, the binary will be merging when it meets the criterion that the orbital angular momentum is less than 3 times the total spin angular momentum, i.e., J_orb < 3J_rot. This suggests that the system will finally merge into a rapid-rotating single star and may produce a luminous red nova. The cyclic oscillation in the O-C diagram can be interpreted by the light-travel time effect (LITE) via the presence of an additional companion.