  The prototype of the Beta Cep class of pulsating stars, Beta Cep, rotates
relatively slowly, and yet displays episodic Halpha emission. Such behaviour is
typical of a classical Be star. For some time this posed a contradiction to our
understanding of the Be phenomena as rapid rotation is thought to be a
prerequisite for the emission phases of Be stars. Recent work has demonstrated
that the Halpha emission is in fact due to a close companion (separation
~0.25") of the star. This resolves the apparent enigma if this close companion
is a classical Be star. We aim to test the hypothesis that this close companion
is a valid Be star by determining properties such as its spectral type and vsin
i. We employed spectroastrometry to investigate the close binary system. Using
the spectroastrometric signatures observed, we split the composite binary
spectra into its constituent spectra in the B band and R band. The split
spectra allow us to estimate spectral types of the binary components. We find
that the primary of the close binary system has a spectral type of B2III and
the secondary a spectral type of B5Ve. From the relationship between mass and
spectral type, we determine the masses of the binary components to be M_p =
12.6+/-3.2 M_* and M_s = 4.4+/-0.7 M_* respectively. The spectroastrometric
data allow some constraint on the orbit, and we suggest a moderate revision to
the previously determined orbit. We confirm that the primary of the system is a
slow rotator (vsin i =29 +43/-29 km/s), while the secondary rotates
significantly faster, at a vsin i =230+/-45 km/s. We show that the close
companion to the beta Cep primary is a valid classical Be star. It has a
spectral type of B5Ve and is a relatively fast rotator.
