semimajor axis help

[monstermunch]

hi, I'm not sure if I am posting in the right place but I could really do with some help figuring out the semi major axis of a binary star system.

I have been give the mean distance of each star from the center of mass (star A = 3.96 AU star B = 6.34 AU)

mean angular separation of 0.74 arcsec

distance of 12.4 pc

orbital period 26.28 years.

Do I use the mean distance of both stars from the center mass and divide by 2 to get the semi major axis or do I use the mean angular separation.

sorry I seem to have jut missed the point on this question so any help would be fab

thanks