docs: def period(A,M1,M2):
      """ 
      calculate binary period from separation. 
  
      Parameters
      ----------
      A : float
          separation A Rsun.
      M1, M2 : float
          M in Msun.
  
      Returns
      -------
      p
          period in days.
  
      """
  
      A *= rsun_cm
      print(A)
      velocity = np.sqrt(grav_const*msun_g*(M1+M2)/A)
      print(old_div(velocity,1.e5))
      
      p = 2.*np.pi * A / velocity
  
      p  /= (60*60*24.)
      return p