It is a close binary composed of a red giant and white dwarf, estimated to be around 2.5 and 0.6 times the mass of the Sun respectively.

[11] The spectrum of the hotter star has changed drastically over 160 years, leading investigators Scott Kenyon and colleagues to surmise that its hotter component, originally a white dwarf, accumulated enough material from the donor giant star to begin burning hydrogen and enlarge and brighten into an A-type white supergiant around 1850.

[8] AG Pegasi has been described as the slowest nova ever recorded,[7] with a constant bolometric luminosity of the hotter star over 130 years from 1850 to 1980.

By the late 20th century, the hotter star has evolved into a hot subdwarf on its way to eventually returning to white dwarf status.

[5] Vogel and colleagues calculated the hotter star must have been accreting material from the red giant for around 5000 years before erupting.