Iota Boötis (ι Boo, ι Boötis) is a member of a binary star system in the constellation Boötes, approximately 96 light-years from Earth. It has the traditional name Asellus Secundus (Latin for "second donkey colt") and the Flamsteed designation 21 Boötis. The companion is HD 234121, a K0 main sequence star.
Iota Boötis has a companion at an angular distance of 38.6 arcseconds, easily separated with binoculars.
The primary component is a white A-type main-sequence dwarf with a mean apparent magnitude of +4.75. It is classified as a Delta Scuti-type variable star and its brightness varies from magnitude +4.73 to +4.78 with a stable period of 38 minutes.
The companion, HD 234121, is a magnitude 7.3 main-sequence star belonging to spectral class K0V. It is separated from ι Boo by . HD 234121 has a mass of, a luminosity of, a temperature of, and a radius of .
The Washington Double Star Catalog lists a third component, a 14th-magnitude star at 90 arcseconds, but it is an unrelated background star.
This star, along with the other Aselli (θ Boo and κ Boo) and λ Boo, were Aulād al Dhiʼbah (أولاد الضّباع - awlād al-ḍibā‘), "the Whelps of the Hyenas".
In Chinese, Chinese: 天槍 (Chinese: Tiān Qiāng), meaning Celestial Spear, refers to an asterism consisting of ι Boötis, κ2 Boötis and θ Boötis. Consequently, the Chinese name for ι Boötis itself is Chinese: 天槍二 (Chinese: Tiān Qiāng èr, English: the Second Star of Celestial Spear).