Spectrum Binary is type of a Binary Star.
A binary star system is either too close or too distant to turn into an optical pair. However, a spectrum of such an object will reveal the spectral fingerprints of two distinct stellar types (if the stars are spectral types distinct).
The drawback with this approach is that because the small, cold stars are more common than the brighter stars, the chances are that the companion is too small to distinguish in a continuum. Also, just detecting two spectrums won’t decide their masses, as relative speeds are required.