Recently, PhD student ZHANG Xudong and Prof. QIAN Shengbang from the Yunnan observatories of the Chinese Academy of Sciences obtained a new orbital period cut-off of contact binaries.

With the help of relevant conditions, they theoretically obtained a lower limit (0.15 days) of orbital period than before.

Contact binaries consist of two strong-interacting component stars. The two components fill their critical Roche lobe and share a common envelope. The formation and evolution of contact binaries is a hot topic in the field of astrophysics. In the past, researchers deemed that the period cut-off of contact binaries is about 0.22 days and there should not be contact binaries with orbital period shorter than 0.22 days.

However, in the past two decades, a large number of contact binaries were discovered by several photometric surveys, more and more short period binaries were discovered. The old period cut-off cannot satisfy the current observation. This problem needs to be solved.

Period-color relation is a very special character of contact binaries. The color of a star reflects the star's surface temperature. Meanwhile, the luminosity of a star, which can also indicate the temperature, is closely related with its mass. Therefore, the orbital period of contact binaries should have some relationship with the massive components.

In this research, ZHANG Xudong and Prof. QIAN compiled a list of 365 contact binaries. These binaries's masses, radii, separation between two components were given. By studying the correlation of their orbital periods, mass ratios, masses and radii, period-mass ratio relation was deduced. Then, a 0.15-day of orbital period was obtained.

At the meantime, in observations, people do have found many contact binaries with orbital period between 0.15 days to 0.22 days. These short-period cool binaries provide us a chance to study the rarity of the extremely short period contact binary, and can provide valuable information on the formation and migration history of low-mass binary stars.