X Cygni is a variable star in the northern constellation of Cygnus, abbreviated X Cyg. This is a Delta Cephei variable that ranges in brightness from an apparent visual magnitude of 5.85 down to 6.91 with a period of 16.386332 days.^[3] At it brightest, this star is dimly visible to the naked eye. The distance to this star is approximately 628 light years based on parallax measurements.^[2] It is drifting further away with a radial velocity of 8.1 km/s.^[5] This star is a likely member of the open cluster Ruprecht 173.^[12]

The variable luminosity of this star was discovered by S. C. Chandler, Jr. in 1886.^[13] In 1907, E. B. Frost showed that X Cyg is an F-type star with a varying radial velocity, behaving analogous to a Delta Cephei variable.^[14] M. Luizet in 1912 found a cyclical pulsation period of 16.38543 days for the variation.^[15] In 1919, F. C. Jordan determined that the color index of the star changed over the course of each cycle, becoming redder as the star grew fainter.^[16] It came to be identified as a member of the benchmark class of stars termed Classical Cepheid variables that satisfy a simple period-luminosity relation.^[17]

In 1954, R. P. Kraft found a stellar classification of F7 Ib at peak brightness,^[18] matching the spectrum of an F-type supergiant star. The class of the star varies over the course of a pulsation cycle, ranging down to G8 Ib at minimum brightness.^[4] R. P. Kraft in 1956 identified a doubling of certain spectral lines, which he explained as the result of a falling shell of matter from a prior pulsation cycle that is colliding with the photosphere.^[19] Unlike most cepheid variables that undergo a single shock per cycle, X Cyg has been found to undergo a double shock.^[20]

No orbiting companion has been identified with a period of ten years or less.^[21] The star shows cirrus with tentative evidence for extended emission of infrared.^[22]