An OH/IR source is a source of both infrared black-body radiation and OH maser electromagnetic radiation. The sources are presumed to be asymptotic giant branch (AGB) stars surrounded by dust (which re-emits the star's light as infrared) and OH (which mases). The size of the cloud and distance to the star can be roughly estimated from features of such a star and its surrounding cloud, which consists of a thin shell of material "coasting" away from the central star. Very-long-baseline interferometry can sometimes determine the angular size of the cloud, and spectrography can sometimes determine the radial velocity of cloud material from the Doppler shift. Timing the variations in the star's emission and the delayed response of the cloud's emissions can yield some absolute distances using the speed of light.