Earlier, we determined that the period of the orbit was 
  about  P = 2.9 days.

  If we assume that the mass of the star is about the same
  as that of our Sun, then

            2             3
           P      =      a

  with "P" in years and "a" in AU.  So,

               P  = 2.9 days  =  (2.9 / 365.25)   =  0.00794  years

  
  Then we can solve for the semi-major axis "a"

                                 2/3
                 a   =   (0.00794)     =   0.040  AU