XO-2 is a binary star system about 490 light-years (150 parsecs) away in the constellation Lynx. It consists of two components, XO-2N and XO-2S, both of which host planetary systems.^[9]

This system is unusual in that the stars are not typically referred to as components A & B, but are designated based on their positions in the sky: XO-2N is the northern star and XO-2S is the southern star.

Both of the stars are slightly cooler than the Sun and are nearly identical to each other. The system has a magnitude of 11 and cannot be seen with the naked eye but is visible through a small telescope. These stars are also notable for their large proper motions.^[7][8]

XO-2N and XO-2S have a separation of approximately 4,600 AU.^[3]

There is one confirmed exoplanet orbiting XO-2N. XO-2Nb, which is classified as a hot Jupiter, was discovered by the XO Telescope using the transit method around XO-2N in 2007. It was initially the only known planet in the system and was referred to as XO-2b.^[3] A long-period variation in the radial velocity of XO-2N was detected in 2015, which could be explained by either a second planet or a stellar activity cycle. The stellar activity explanation is considered more likely,^[6] and is further supported by a 2024 study.^[10]

Two planets were reported to orbit around XO-2S in 2014 using the radial velocity method. One of them is Jupiter-mass and another has a mass comparable to Saturn.^[9] A 2024 study found evidence for a third, super-Jupiter mass planet around XO-2S.^[10]

• HD 20781 and HD 20782
• HD 133131
• Lists of exoplanets