Kerr-Sen dilaton-axion black hole lensing in the strong deflection limit

In the present work we study numerically quasiequatorial lensing by the charged, stationary, axially symmetric Kerr-Sen dilaton-axion black hole in the strong deflection limit. In this approximation we compute the magnification and the positions of the relativistic images. The most outstanding effect is that the Kerr-Sen black hole caustics drift away from the optical axis and shift in the clockwise direction with respect to the Kerr caustics. The intersections of the critical curves on the equatorial plane as a function of the black hole angular momentum are found, and it is shown that they decrease with the increase of the parameter Q(2)/M. All of the lensing quantities are compared to particular cases as Schwarzschild, Kerr, and Gibbons-Maeda black holes.