Composite spectrum hot subdwarf binaries

The formation of hot subdwarf stars (sdB) is closely related to binary evolution on the red giant branch. Observations show that hot subdwarfs are either in close binaries with low mass K-M type or white dwarf companions, or in wide binaries with more massive F-G type companions. A small fraction is most strikingly single. In close binaries the radiative interactions give rise to the reflection effect and produces a variable composite spectrum. More massive companions bear similar optical luminosities as sdB stars and therefore double-lined spectra are observed. To investigate such systems in detail and precision spectral decomposition/disentangling is needed. I describe our wavelength space disentangling method and bring examples why such an approach to composite spectra offers a potential to find the history of hot subdwarfs.