The first photometric and orbital period investigation of an extremely low mass ratio contact binary with a sudden period change TYC 4002-2628-1

Photometric observations for the totally eclipsing binary system TYC 4002-2628-1, were obtained between November 2020 and November 2021. To determine the stellar atmospheric parameters, a spectral image was taken with the 2.16 m telescope at National Astronomical Observatory of China (NAOC). TYC 4002-2628-1 is a low-amplitude (about 0.15 mag for V band) short-period (0.3670495 d) contact eclipsing binary with a total secondary eclipse. Intrinsic light curve variations and the reversal of the O'Connell effect are detected in the light curves, which may be due to spot activity. Based on the photometric solutions derived from the multiband time series light curves, TYC 4002-2628-1 is an extremely low-mass ratio contact binary with a mass ratio of q similar to 0.0482 and a fill-out factor of f similar to 5 per cent. By analysing the O - C variations, we find that its orbital period remains unchanged when BJD < 2458321. Then the orbital period changed suddenly around BJD 2458743 and has an increasing rate of dP/di = 1.62 x 10(-5) d.yr(-1) = 140 s.century(-1). If confirmed, TYC 4002-2628-1 would be the contact binary with the highest orbital period increasing rate so far. By investigating the ratio of orbital angular momentum to the spin angular momentum (J(orb)/J(spin )< 3), the instability mass ratio (q(inst)/q = 1.84) and the instability separation (A(inst)/A = 1.35), TYC 4002-2628-1 can be regarded as a merger candidate.