Perturbative approach to the astrometric microlensing due to an   extrasolar planet
  We have developed a perturbative approach to microlensing due to an extrasolar planetary lens. In particular, we have found analytic formulae for triple images. We have used the formulae to investigate the astrometric microlensing due to the extrasolar planetary lens, in expectation of dramatic improvements in the precision of the future astrometric measurements. For a weak lensing case, we have shown how the maximum angular size and the typical time scale of the anomalous shift of the lightcentroid are dependent on the mass ratio and angular separation between the star and the planet. 
Hideki Asada