  Context: The technique of matching synthetic spectra computed with
theoretical stellar atmosphere models to the observations is widely used in
deriving fundamental parameters of massive stars. When applied to binaries,
however, these models generally neglect the interaction effects present in
these systems Aims: The aim of this paper is to explore the uncertainties in
binary stellar parameters that are derived from single-star models Methods:
Synthetic spectra that include the tidal perturbations and irradiation effects
are computed for the binary system alpha Virginis (Spica) using our
recently-developed CoMBiSpeC model. The synthetic spectra are compared to
S/N~2000 observations and optimum values of Teff and log(g) are derived.
Results: The binary interactions have only a small effect on the strength of
the photospheric absorption lines in Spica (<2% for the primary and <4% for the
secondary). These differences are comparable to the uncertainties inherent to
the process of matching synthetic spectra to the observations and thus the
derived values of Teff and log(g) are unaffected by the binary perturbations.
On the other hand, the interactions do produce significant phase-dependent line
profile variations in the primary star, leading to systematic distortions in
the shape of its radial velocity curve. Migrating sub-features ("bumps") are
predicted by our model to be present in the same photospheric lines as
observed, and their appearance does not require any a priori assumptions
regarding non-radial pulsation modes. Matching the strength of lines in which
the most prominent "bumps" occur requires synthetic spectra computed with
larger "microturbulence" than that required by other lines.
