The Blazhko effect is a secondary variation of the amplitude and period of some RR Lyrae stars and related pulsating variables, named for its discoverer, the Russian astronomer Sergei Blazko (1870–1956). The effect is displayed, for example, by the RR Lyrae star XZ Cygni, which has a primary pulsation period of about 0.466 days but displays a further modulation with a 57-day cycle. Although the Blazhko effect is not properly understood, it is thought to involve double-mode pulsation or, in some cases, an oblique rotator, in which the spin axis and magnetic axis of the star are not aligned.