Rotating variable stars' luminosity varies because either they have a non-uniform surface brightness and/or ellipsoidal shapes (not a 'perfect' sphere). The non-uniformity of a star can be due to spots (like larger sun-spots) which can be caused by thermal or chemical inhomogeneity (meaning uneven distribution) in the atmosphere. The changes in brightness that we observe are not because of physical changes to the star, but what we actually see of the star due to its rotation.
Alpha2 Canum Venaticorum variables are main-sequence stars. They have strong magnetic fields that vary, along with brightness, in a period of about 0.5-160 days. The amplitudes of the brightness changes are usually around 0.1-1 magnitudes.NS Del
Rotating ellipsoidal binary variables are close binary systems with ellipsoidal components, which change combined brightnesses with periods equal to those of orbital motion because of the change in which part of the stars is being seen by an observer. The components do not eclipse each other.IV Vir
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