A type of cataclysmic binary whose light exhibits linear and, more significantly, extremely strong circular polarization (hence the alternative name polar); abbr. AM. The polarization varies smoothly over the orbital period. Such systems consist of a K–M dwarf and a strongly magnetic compact object, accretion taking place down an accretion column over the latter’s magnetic poles. The compact star’s rotation is synchronized with the orbital period and, in the case of AM Her itself, the system shows eclipses of up to 0.7 mag. every 3.1 hours. Optical variations in the form of flickering, with amplitudes up to 0.8 mag., are also seen. The long-term light curves also show ‘high’ and ‘low’ states, which in the case of the type star range from mag. 12.3 in its ‘high’ state to 15.7 in its ‘low’ state. These systems also exhibit X-ray variability.