31 Cygni
31 Cygni, also known as ο1 Cygni, Omicron1 Cygni, or V695 Cygni, is a star in the constellation Cygnus.
The Bayer designation ο has been variously applied to two or three of the stars 30, 31, and 32 Cygni. 31 Cygni has been designated, variously, as ο1 or ο2 Cygni — therefore for clarity, it is preferred to use the Flamsteed designation 31 Cygni.
It is an Algol-type eclipsing binary and ranges between magnitudes 3.73 and 3.89 over a period of ten years. The component stars are an orange supergiant of spectral type K4Iab and a blue-white star likely to be evolving off the main sequence with a spectral type of B4IV-V. The eclipsing system has been studied in attempts to determine an accurate direct mass for a red supergiant. The value is believed to be accurate to about 2%, but there are some discrepancies in the orbital fit.
30 Cygni is another naked eye star a tenth of a degree away, forming a bright triple.
32 Cygni is about a degree away to the north, also a detached eclipsing binary system. It comprises a large cool evolved star and a small hot main sequence or subgiant companion.
