井宿增二

井宿增二（RT Aur）是御夫座的一颗恒星。

井宿增二是F-型黄超巨星，平均视星等 +5.75，距离地球大约1,561光年。它是属于造父变星的变星，变光范围从+5.00至+5.82，周期3.73天。位于银经183.15，银纬8.92，其B1900.0坐标为赤经6h 22m 8.4s，赤纬+30° 8.92′ 18″。

RT Aurigae (RT Aur, 48 Aurigae) is a yellow supergiant variable star in the constellation Auriga, about 1,500 light years from Earth. Although its brightness is variable, it is consistently visible to the naked eye under good observing conditions.

RT Aurigae is an F to G type Classical Cepheid variable which varies from magnitude +5.00 to +5.82 with a period of 3.728309 days. The variability was discovered in 1905, by an English schoolmaster and amateur astronomer, Thomas Hinsley Astbury. It was quickly recognised as a member of the class of Cepheid variables, but their nature was not understood at that time. Radial velocity changes were detected corresponding to the brightness variations, but the idea that these were caused by stellar pulsations and temperature changes was largely dismissed in favour of orbital motions of a binary star…

RT Aurigae has been suspected to be a spectroscopic binary system, but this has not been confirmed. The strongest evidence was found in 2013 using CHARA array optical interferometry. The companion would be 6.7 magnitudes fainter than the supergiant primary, cooler and fainter than an F0 main sequence star. The two stars are separated by 2.1 milli-arc seconds.