ZZ Ceti star
A ZZ Ceti star is a white dwarf that is also a pulsating variable and changes in brightness by 0.001 to 0.2 magnitude over a period of 30 seconds to 25 minutes. Non-radial pulsations are involved with multiple periodicities, which result in only slight changes in the star's size but significant fluctuations in temperature. There are three subtypes: ZZA with DA (hydrogen absorption) spectra, ZZB with DB (helium absorption spectra), and ZZO, sometimes called GW Virginis stars, with extremely hot, DO spectra. Sometimes flares of about one magnitude are observed, but these may be due to UV Ceti type companion stars.