A close binary containing two evolved stars neither of which has yet overflowed its Roche lobe so that the pair are still a detached system; abbr. AR. They belong to the RS Canum Venaticorum class of binary. The rotational period of the stars is locked to the orbital period, resulting in enhanced activity from both components. Enhanced versions of solar-like flares excite the coronae of the stars, producing light at extreme ultraviolet and X-ray wavelengths, while the interaction of electrons and magnetic fields gives rise to radio emission. AR Lacertae itself is also an eclipsing binary, varying between magnitudes 6.1 and 6.8 every 1.98 days.