 Unlike true orbital velocity, radial velocity can be determined from Doppler spectroscopy of spectral lines in the light of a star, or from variations in the arrival times of pulses from a radio pulsar.A binary system is called a single-lined spectroscopic binary if the radial motion of only one of the two binary components can be measured.In this case, a lower limit on the mass of the "other" (unseen) component can be determined.