of the outer disk. For instance, a binary system with a 30 AU separation is expected to initially have individual disks that are ∼ 10-15 AU in radius. Given that the viscous timescale is roughly proportional to the size of the disk, such small disks are likely to have very small accretion lifetimes, smaller than the age of the sample. 

For systems with binary separations much smaller than the size of a typical disk, the outer disk can survive in the form of a circumbinary disk, with a tidally truncated inner hole with a radius ∼ 2 × the orbital separation (Artymowicz & Lubow, 1994). Such systems could be classified as transition disks based on their SEDs. We do not see an increased incidence of binaries with separations in the ∼ 8-20 AU range, the range where we are sensitive to companions that could in principle carve the inferred inner holes. In fact, the incidence of binaries in this separation range is lower for our transition disks than it is for the samples of both non-transition disks and disk-less stars. This suggests that stellar companions at 8-20 AU separations are not responsible for a large fraction of the transition disk population. Given the size of our transition disk sample, however, this result carries little statistical weight, and should be considered only a hint. According to a KS test, the distribution of binaries separations of transition objects is indistinguishable from that of both non-transition disks (p = 28%) and disk-less stars (p = 12%). On the other hand, the fact that 2 of our transition disk targets have previously known companions at sub-AU separations suggests that circumbinary disks around very tight systems (e.g., r ≲ 1-5 AU) could indeed represent an important component of the transition disk population. Unfortunately, the presence of tight companions (r ≲ 8 AU) remains completely unconstrained for most of our sample. A complementary radial velocity survey to find the tightest companions is highly desirable to firmly establish the fraction of transition disks that could be accounted for by close binaries. 

