  We present a new method for probabilistic generative modelling of stellar
colour-magnitude diagrams (CMDs) to infer the frequency of binary stars and
their mass-ratio distribution. The method invokes a mixture model to account
for overlapping populations of single stars, binaries and outliers in the CMD.
We apply the model to Gaia observations of the old open cluster, M67, and find
a frequency $f_B(q > 0.5) = 0.258 \pm 0.019$ for binary stars with mass ratio
greater than 0.5. The form of the mass-ratio distribution function rises
towards higher mass ratios for $q > 0.3$.
