R Canis Majoris is an eclipsing interacting binary star system in the constellation

Constellation

In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....

 Canis Major

Canis Major

Canis Major is one of the 88 modern constellations, and was included in the 2nd-century astronomer Ptolemy's 48 constellations. Its name is Latin for 'greater dog', and is commonly represented as one of the dogs following Orion the hunter...

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Eclipse timings

Eclipse timings for R Canis Majoris have been being measured since 1887, and whilst at present the time period appears constant at 1.1359 days, periodic quasi-sinusoidal variations of the eclipse arrival times have been taking place with a periodicity of around 93 years. This has led to the suggestion that there exists a third non-eclipsing body in the system whose gravitational pull is responsible for these variations.

Interacting binary star

R Canis Majoris is thought to be an interacting binary star. The secondary star has exceeded its Roche lobe

Roche lobe

The Roche lobe is the region of space around a star in a binary system within which orbiting material is gravitationally bound to that star. If the star expands past its Roche lobe, then the material can escape the gravitational pull of the star. If the star is in a binary system then the material...

 and accreted mass to the primary star. This has resulted in the early evolution of the secondary star into the subgiant branch, and increased helium rich material in the primary, causing it to burn brighter and have a higher effective temperature

Effective temperature

The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation...

than would usually be expected for a star of its mass.