Sigma Geminorum (σ Gem) is a binary star[9] system in the constellation Gemini, just to the northwest of Pollux.

The stellar luminosity shows indications of ellipsoidal variation, as the primary component is partly filling its Roche lobe due to gravitational interaction between the two stars.

The surface of the primary has large star spots that are locked onto the face oriented toward the secondary component.

[9] The surface activity makes the star a bright X-ray emission source[11] with a luminosity of 119.41×1029 ergs s−1.

[8] It shines with 39[8] times the solar luminosity from its outer atmosphere at an effective temperature of 4571 K.[6] It is roughly 5[8] billion years old.