Strong lensing constraints on modified gravity models.

We impose the first strong-lensing constraints on a wide class of modified gravity models where an extra field that modifies gravity also couples to photons (either directly or indirectly through a coupling with baryons) and thus modifies lensing. We use the nonsingular isothermal ellipsoid (NIE) profile as an effective potential, which produces flat galactic rotation curves. If a concrete modified gravity model gives a flat rotation curve, then the parameter $\Gamma$ that characterizes the lensing effect must take some definite value. We find that $\Gamma = 1.24\pm0.65$ at $1\sigma$, consistent with general relativity ($\Gamma = 1$). This constrains the parameter space in some recently proposed models.