Semidetached Binaries - Designations - Hot and Cold

Hot and Cold

The components of a binary star system may be designated by their relative temperatures as the hot companion and cool companion.

Examples:

• Antares (Alpha Scorpii) is a red supergiant star in a binary system with a hotter blue main sequence star Antares B. Antares B can therefore be termed a hot companion of the cool supergiant.
• Symbiotic stars are binary star systems composed of a late-type giant star and a hotter companion object. Since the nature of the companion is not well-established in all cases, it may be termed a "hot companion".
• The luminous blue variable Eta Carinae has recently been determined to be a binary star system. The secondary appears to have a higher temperature than the primary and has therefore been described as being the "hot companion" star. It may be a Wolf-Rayet star.
• R Aquarii shows a spectrum which simultaneously displays both a cool and hot signature. This combination is the result of a cool red supergiant accompanied by a smaller, hotter companion. Matter flows from the supergiant to the smaller, denser companion.
• NASA's Kepler mission has discovered examples of eclipsing binary stars where the secondary is the hotter component. KOI-74b is a 12,000 K white dwarf companion of KOI-74 (KIC 6889235), a 9,400 K early A-type main sequence star. KOI-81b is a 13,000 K white dwarf companion of KOI-81 (KIC 8823868), a 10,000 K late B-type main sequence star.