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An Ap star belongs to a subtype of A-type stars, that have more than expected of some metals, such as strontium, chromium, europium, praseodymium, and neodymium ("p" is probably for peculiar). They also commonly have a slow rotation rate and a strong magnetic field. Bp stars are analogous B-type stars. A roAp star (for rapidly oscillating Ap star) is a sub-type, a pulsating star with an oscillation period in minutes.