Stellar models of massive single stars are still plagued by major
uncertainties. Testing and calibrating against observations is essential for
their reliability. For this purpose one preferably uses observed stars that
have never experienced strong binary interaction, i.e. "true single stars".
However, the binary fraction among massive stars is high and identifying "true
single stars" is not straight forward. Binary interaction affects systems in
such a way that the initially less massive star becomes, or appears to be,
single. For example, mass transfer results in a widening of the orbit and a
decrease of the luminosity of the donor star, which makes it very hard to
detect. After a merger or disruption of the system by the supernova explosion,
no companion will be present.
The only unambiguous identification of "true single stars" is possible in
detached binaries, which contain two main-sequence stars. For these systems we
can exclude the occurrence of mass transfer since their birth. A further
advantage is that binaries can often provide us with direct measurements of the
fundamental stellar parameters. Therefore, we argue these binaries are worth
the effort needed to observe and analyze them. They may provide the most
stringent test cases for single stellar models.Comment: 5 pages, 1 figure, contribution to the proceedings of "The
multi-wavelength view of hot, massive stars", 39th Li`ege Int. Astroph.
Coll., 12-16 July 201