OK, something is moving in between us and the lensing object - and it must be very close to us because the lag is zero. Moreover, because the images are separated by 6 arc seconds which is a rather large angle, the nearby object(s) causing the oscillations must be pretty large. Because a hypothetical binary star or a similar object would have to be unacceptably heavy to give you the speedy oscillations, the source of the additional variations should be an oscillating cosmic string, they say. A string is able to oscillate very quickly.