Arkab, or Beta Sagittarii, is really two stars, separated by only 0.36° in the sky. The western and brighter star is Arkab Prior or β1 Sgr, the leader as the pair goes across the sky; the eastern follower is Arkab Posterior or β2 Sgr. Both only magnitudes 4.01 and 4.29, respectively, and lie far to the south, in the leg of the Archer: their traditional name Arkab comes from the Arabic "carqub" meaning "hamstring". In his 1602 star atlas "Uranometria," however, Bayer - who assigned them the β designation - seems to have been fooled into thinking that these stars are brighter than they really are, perhaps because of their low position and an overestimation of dimming by the Earth's thick atmosphere.
Arkab Prior
In spite of their proximity in the sky, Arkab Prior and Posterior are not a real couple. Arkab Prior 378 lies light years away, and Arkab Posterior at 139 light years - only about a third as far. Both Arkabs are understudied. Arkab Prior's listed temperature of 13,630 K is inconsistent with its B9 V spectral class (which implies more like 11,000 K). Adopting the lower value, its magnitude and distance give it a luminosity 440 times the Sun's, a radius 6 times solar, and a mass of 3 suns, and is near the end of its hydrogen-fusing lifetime.
Arkab Prior has a dimmer (magnitude 7.11) companion 28" away. This is a hydrogen-fusing, class A5 V main-sequence star of 1.8 solar masses. It lies at least 3300 AU away, taking at least 82,000 years to orbit. From β1 Sgr A (the brighter), the companion would shine with the brightness of our full Moon, whereas from β1 Sgr B (the companion), the dominating star would appear 17 times brighter.
Arkab Posterior
Arkab Posterior is a classed as an F2 III giant with a temperature of 6900 K, and a luminosity of 27 suns. Luminosity and temperature give it a radius 1.4 times solar, and a mass twice solar. However, from stellar evolutionary theory, Arkab Posterior is clearly not a giant, but a main-sequence star in the end stages of core hydrogen fusion. At best it's a subgiant, with an age of about 1.2 billion years. (The Sun's five billion year age is the result of its lower mass and fusion rate.)
β2 Sgr's rapid projected rotation velocity of 126 km/sec gives it a rotation period under 1.4 days. As an F2 star, Arkab Posterior is just a bit warmer than "rotation break" at which stars turn faster quickly. The reason is not that warmer stars are particularly spun up, but that cooler stars like the Sun are spun down by their magnetic fields, which drag on the solar winds.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]