A type of variable white dwarf that undergoes non-radial pulsation, with minimal variation in size but significant temperature changes; abbr. ZZ. Periods range from 30 s to 25 min, and generally several periods exist simultaneously. Amplitudes are 0.001–0.2 mag. 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 showing HeII and and CIV absorption lines. Some ZZ Ceti stars exhibit flares, thought to be caused by a companion UV Ceti star. ZZ Ceti itself is of 14th magnitude and type ZZA.