We present evidence for ultraviolet/optical microlensing in the gravitationally lensed quasar Q 0957+561 .
We combine new measurements from our optical monitoring campaign at the United States Naval Observatory , Flagstaff ( USNO ) with measurements from the literature and find that the time-delay-corrected r band flux ratio m _ { A } - m _ { B } has increased by \sim 0.1 magnitudes over a period of five years beginning in the fall of 2005 .
We apply our Monte Carlo microlensing analysis procedure to the composite light curves , obtaining a measurement of the optical accretion disk size , \log \ { ( r _ { s } / \textrm { cm } ) [ \cos ( i ) / 0.5 ] ^ { 1 / 2 } \ } = 16.2 \pm 0.5 , that is consistent with the quasar accretion disk size – black hole mass relation .