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A spinning star is throwing off stellar wind, and electromagnetic radiation, which might be carrying away angular momentum, so that the star loses angular momentum, and its angular momentum per unit mass may get reduced as result. I'm wondering whether gravitational waves or something else might also carry away angular momentum from the star. Maybe the planets are able to get some of the stars spin angular momentum.

Then if the star sheds a nebula near the end of its life, that nebula will rotate slower than the nebula that the star formed from in the first place. This might be the only way a naturally occurring siderostat could form.

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The main way that a star loses angular momentum is through a magnetised stellar wind. The highly ionised wind couples to the large scale magnetic field. This coupling is maintained out to some radius (known as the Alfven radius) that depends on the strength and geometry of the magnetic field and the physical conditions in the wind.

Prior to decoupling, the wind is forced into approximate co-rotation with the stellar surface, where the footpoints of the magnetic field are anchored. After decoupling, the wind then carries away angular momentum, enhanced by the lever arm offered by the Alfven radius. Angular momentum loss is much less efficient in stars with weak magnetic fields - most stars that are hotter than the Sun, and massive stars tend to keep lots of angular momentum into "old age".

The other mechanisms you mention are inefficient by comparison. Spinnng objects with axial symmetry do not emit gravitational waves. Planets are usually too far away for tidal effects to efficiently couple the star to the planet. The exception can be for "hot Jupiters" that can get close enough that there could be a tidal transfer of angular momentum between a close-in planet and a very rapidly rotating star. This can also work in the opposite direction if the planet's orbital period is shorter than the stellar rotation period.

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