Context : The prototype of the \mathrm { \beta } Cephei class of pulsating stars , \mathrm { \beta } Cep , rotates relatively slowly , and yet displays episodic \mathrm { H } \alpha emission .
Such behaviour is typical of a rapidly rotating , 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 characteristic emission phases of Be stars .
Recent work has demonstrated that the \mathrm { H } \alpha emission is in fact due to a close companion ( separation \mathrm { \approx } 0.25 \arcsec ) of the star .
This resolves the apparent enigma if this close companion is indeed a classical Be star , as has been proposed .

Aims : We aim to test the hypothesis that this close companion is a valid Be star by determining properties such as its spectral type and v \sin i .

Methods : We employed the technique of 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 ( \mathrm { 4 } 200 - 5000 \AA ) and R band ( \mathrm { 6 } 200 - 7000 \AA ) .

Results : The spectroastrometrically 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 \mathrm { M } _ { \mathrm { pri } } = 12.6 \pm 3.2 M _ { \sun } and \mathrm { M } _ { \mathrm { sec } } = 4.4 \pm 0.7 M _ { \sun } 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 ( v \sin i = \mathrm { 2 } 9 ^ { +43 } _ { -29 } \mathrm { km s ^ { -1 } } ) , while the secondary rotates significantly faster , at a v \sin i = \mathrm { 2 } 30 \pm 45 \mathrm { km s ^ { -1 } } .

Conclusions : We show that the close companion to the \mathrm { \beta } Cephei primary is certainly a valid classical Be star .
It has a spectral type of B5Ve and is a relatively fast rotator .
We confirm that the \mathrm { \beta } Cephei system does not contradict our current understanding of classical Be stars .