RR Lyrae star WORD ORIGIN noun Astronomy. one of a group of pulsating variable stars found in the halo of the Milky Way, with periods between 1.2 and 30 hours; all have approximately the same luminosity and are therefore used for measuring distances. Liberaldictionary.com
RR Lyrae stars have a sharp increase in brightness and a more gradual dimming, or as one would describe the waveform as having fast attack and slow decay.
H R diagram showing the NGC 5053 RR Lyrae stars, HB tracks and ZAHB (½Feu003dHŠ ¼ À2:26 dex, ½Ou003dFeŠ ¼ 0:75 dex) from Dorman (1992), and theoretical pulsation