  Binaries are very common in galaxies, and more than half of Galactic hot
subdwarf stars, which are thought as a possible origin of UV-upturn of old
stellar populations, are found in binaries. Previous works showed that binary
evolution can make the spectra of binary star populations significantly
different from those of single star populations. However, the effect of binary
evolution has not been taken into account in most works of spectral fitting of
galaxies. This paper studies the role of binary evolution in spectral fitting
of early-type galaxies, via a stellar population synthesis model including both
single and binary star populations. Spectra from ultraviolet to optical band
are fitted to determine a few parameters of galaxies. The results show that the
inclusion of binaries in stellar population models may lead to obvious change
in the determination of some galaxy parameters and therefore it is potentially
important for spectral studies. In particular, the ages of young components of
composite stellar populations become much older when using binary star
population models instead of single star population models. This implies that
binary star population models will measure significantly different star
formation histories (SFHs) for galaxies compared to single star population
models. In addition, stellar population models with binary interactions measure
larger dust extinctions than single star population models on average. It
suggests that when using binary star population models instead of single star
population models, negative extinctions are possibly unnecessary in spectral
fitting of early-type galaxies.
