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arXiv: A Spectroscopic Orbit for Regulus - apparently Regulus is a spectroscopic binary: the companion is likely a white dwarf with semimajor axis 0.35 AU. This would explain the rapid rotation rate of the primary star through mass transfer when the companion was a red giant.

A study of the Past and Future History of Regulus taking into account the white dwarf companion and the implications for mass transfer.

One of the main implications is that Regulus is probably older than previously thought, perhaps around 1 Gyr. The future evolution will involve mass transfer from Regulus to the white dwarf, resulting in a common envelope phase. After that, there are several possibilities...