We study the formation of dust in the expanding gas ejected as a result of a common envelope binary interaction. In our novel approach, we apply the dust formation model of Nozawa et al. to the outputs of the 3D hydrodynamic SPH simulation performed by Iaconi et al., that involves a giant of 0.88~ms and 83~rs, with a companion of 0.6~ms placed on the surface of the giant in circular orbit. After simulating the dynamic in-spiral phase we follow the expansion of the ejecta for $simeq 18,000$~days. During this period the gas is able to cool down enough to reach dust formation temperatures. Our results show that dust forms efficiently in the window between $simeq 300$~days (the end of the dynamic in-spiral) and $simeq 5000$~days. The dust forms in two separate populations; an outer one in the material ejected during the first few orbits of the companion inside the primarys envelope and an inner one in the rest of the ejected material. We are able to fit the grain size distribution at the end of the simulation with a double power law. The slope of the power law for smaller grains is flatter than that for larger grains, creating a knee-shaped distribution. The power law indexes are however different from the classical values determined for the interstellar medium. We also estimate that the contribution to cosmic dust by common envelope events is not negligible and comparable to that of novae and supernovae.
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