An RR Lyrae variable (or RR Lyrae star or just RR Lyrae) is a type of variable star with a predictable pulsation period so it can serve as a standard candle. They are thought to be former A-type or F-type stars in the horizontal branch, which places them in the instability strip. Their pulsation period appears to be related to their absolute magnitude.
They are naturally named after the prototype star, RR Lyrae. They are useful for determining the distance to Milky-Way globular clusters, where they are common, but not sufficiently bright to be of much use for determining distance to other galaxies.