Annu. Rev. Astron. Astrophys. 1989. 27: 139-59 Copyright © 1989 by Annual Reviews. All rights reserved

4.2 RR Lyrae Stars

From both theoretical arguments and observational evidence (they occur only in truly old clusters in the MWG and the Magellanic Clouds), RR Lyrae stars should be found only among 15-Gyr-old stars. This criterion is sufficient but not necessary; there are bona fide old Galactic globular clusters without RR Lyrae stars, and, of course, very old metal-rich stars may not become normal RR Lyrae stars.