One subclass of these stars, d, consists of double
One subclass of these stars, d, consists of double-mode pulsating stars, and that subclass is commonly plotted on Petersen diagrams to analyze the effects of metallicity on their properties. Commonly found in globular clusters, the  luminosities of these Population II stars are typically around forty five solar, and their periods vary from a few hours to two days. Formerly called cluster Cepheids, FTP, identify this class of pulsating horizontal branch stars, named for a star in the Lyre.
One subclass of these stars, d, consists of double-mode pulsating stars, and that subclass is commonly plotted on Petersen diagrams to analyze the effects of metallicity on their properties.
{RR Lyrae} variables