  After the amazing discoveries by the GRAVITY collaboration in the last few
years on the star S2 orbiting the black hole Sgr A* in the center of the Milky
Way, we present a detailed investigation of the impact of gravitational lensing
on the reconstruction of stellar orbits around this massive black hole. We
evaluate the lensing astrometric effects on the stars S2, S38 and S55 and how
these systematically affect the derived orbital parameters. The effect is below
current uncertainties, but not negligible. With the addition of more
observations on these stars, it will be possible to let the astrometric shift
by lensing emerge from the statistical noise and be finally detected.
  By repeating the analysis on a smaller semimajor axis $a$ and various
inclinations $i$, we are able to quantify the lensing effects on a broader
range of parameters. As expected, for smaller semimajor axes and for nearly
edge-on orbits lensing effects increase by about an order of magnitude.
