  Supernovae (SNe) should both frequently have a binary companion at death and
form significant amounts of dust. This implies that any binary companion must
lie at the center of an expanding dust cloud and the variable obscuration of
the companion as the SN remnant (SNR) expands will both unambiguously mark the
companion and allow the measurement of the dust content through absorption
rather than emission for decades after the explosion. However, sufficiently hot
and luminous companions can suppress dust formation by rapidly photo-ionizing
the condensible species in the ejecta. This provides a means of reconciling the
Type IIb SNe Cas A, which lacks a luminous companion and formed a significant
amount of dust (Md > 0.1 Msun), with the Type IIb SNe 1993J and 2011dh, both of
which appear to have a luminous companion and to have formed a negligible
amount of dust (Md < 0.001 Msun). The Crab and SN 1987A are consistent with
this picture, as both lack a luminous companion and formed significant amounts
of dust. An unrecognized dependence of dust formation on the properties of
binary companions may help to explain why the evidence for dust formation in
SNe appears so contradictory.
