Bi-periodic variation in the BL Lac AO 0235+164

High resolution VLBI hybrid map of the BL Lacertae object AO 0235+164 has been produced at a wavelength of 6 cm. The map shows that the object's radio structure is dominated by a strong, nearly unresolved core with a weak and clear component in northeast direction and a faint one in southwest direction. The positional angle of its jet component are equal to66.4^°, which is the biggest one in comparison with previous results. Based on the variation of its flux density with time at three different frequencies, we find that the flux density of AO 0235+164 shows bi-periodic variation, i.e., the shorter periodic variation of ∼ 1.81 years and a longer periodic variation of ∼ 3.63 years. The later is essentially in agreement with our earlier predicted results that the existence of the periodic variation of ∼ 3.63 years may be caused by the precession of its `central engine'. This bi-periodic variation is probably the results of the joint action of jet outbursts and jet rotation. With the binary black hole models of Kaastra and Roos, we get the minimum total mass of the binary system of 1.46 × 10⁸ M _⊙. This revised version was published online in July 2006 with corrections to the Cover Date.