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tokens: values of u 0 , are low. For rel , our prior is flat between 0 and 10masyr 1 (see Henderson 2015 ). The priors for and f , respectively, are flat from 0 to and 0 to 2rad. For inclination, the prior is flat in cos from 1 to 1. Note that while flat priors for some of our parameters are unrealistic, the assumptions are reasonable, given poor constraints and the resulting ease of computation. Since all of the priors are flat, the prior term can be pulled out of the integral in ( 3 ) as a factor of p V 1 , where V i ( i i ) is the volume of the priors. i is the upper boundary of the prior for parameter i and i is the lower boundary. Using the Fisher matrix approximation, the evidence is then We use Monte Carlo integration to compute the evidence integrals. 4 RESULTS The test in Bayesian statistics of support for one model over another is the Bayes factor, which is the ratio of the binary-lens evidence to the single-lens evidence: B E B / E S . We apply the scale from Jeffreys ( 1961 ) to determine support, where ln B 5 is strong evidence, 2.5 ln B 5 is moderate evidence, 1 ln B 2.5 is weak evidence and ln B 1 is inconclusive. In the limits that q 0 and s 0 0, the peak likelihood ratio 1, while ln B becomes negative, indicating that the single-lens model is favoured. We compute whether a BBH system with mass parameters similar to those of the first detected LIGO event, GW150914, would be detectable by microlensing in our own Galaxy. Our fiducial model is M 65M , q 0.8, rel 5.5masyr 1 and u 0 0.4 Einstein radii. We choose a lens distance D L 4kpc and a source star in the Galactic Bulge at D S 8kpc with apparent magnitude I 15, on the bright end of typical source magnitudes in current microlensing surveys. For this model, we find that BBH separations down to approximately s 0 0.0250 Einstein radii are detectable with strong evidence, given ideal observing conditions for one year around the peak of the event. For a dimmer source star with I 17, separations down to only s 0 0.0270 Einstein radii are detectable. We also calculate the 2 between the binary and single lenses, both for our fiducial separation s 0 0.01 Einstein radii and the minimum detectable separation s 0 0.0250 Einstein radii. For the fiducial case, ln B 7.65 corresponds to 2 1.36; for the minimum detectable s 0 0.0250 Einstein radii, ln B 5 and 2 53.4. The approximate merger time of BBHs is (see Peters Mathews 1963 ) as follows: where c is the speed of light and G is the gravitational constant. Therefore, BBHs that fit our fiducial model with a separation of s 0 0.0250 Einstein radii will merge in about 2.0810 3 Gyr, which is much greater than the 13.8Gyr age of the Universe. BBHs formed with these parameters in the early Universe can serve only as present-day GW sources if their initial separation was s 0 0.00716 Einstein radii, 3.5times smaller than the minimum separation detectable by KMTNet. The dependence of this minimum separation on the binary parameters is explored in Fig. 2 ,