V3903 Sagittarii

V3903 Sagittarii (also known as 11 Sagittarii, or its abbreviation 11 Sgr) is an eclipsing binary star system in the constellation Sagittiarus. It creates an H II region LBN 29 (Sh 2-29) 1070 pc away from the Sun.

John Flamsteed designated this star the Flamsteed designation 11 Sagittarii, although the designation 11 Sgr is now more often used to refer to the star he designated 1 Sagittarii.

Properties
The system consists of two hot main-sequence stars. The spectral types have historically been accepted as O7V and O9V, but the more recent Galactic O-Star Spectroscopic Survey gives a spectral type of O7V(n)z + B0:V:. The system is one of the youngest-known eclipsing binaries, and one of the few containing such massive stars that have not yet filled their roche lobes. Their likely age is around 1.6 million years, and they have current masses of and.

V3903 Sagittarii varies in brightness. The General Catalogue of Variable Stars lists it as a possible hot irregular Orion variable star, but it has been shown to be a detached eclipsing binary system. The two stars are detached, that is they do not fill their roche lobes, which makes it an Algol-type eclipsing variable. The period is one day, 18 hours, 52 minutes.

The distance derived from the annual parallax measured by the Hipparcos satellite is around 459 pc, but the distance calculated from the physical properties of the two stars is 1,500 pc. Later measurements have resulted in refined distances estimates of 1,070 pc and 1,200 pc.