lack of accretion is a particularly good indicator of a low disk mass (See Figure 6). One the other hand, Najita et al. (2006) drew their sample from the Spitzer spectroscopic survey of Taurus presented by Furlan et al. (2006), which in turn selected their sample based on mid-IR colors and fluxes from the Infrared Astronomical Satellite (IRAS). As a result, the Spitzer spectroscopic survey of Taurus was clearly biased towards the brightest objects in the mid-IR. This helps to explain the high disk masses obtained for the transition disks studied by Najita et al. (2006), as objects with strong mid-IR excesses tend to have higher disk masses (Figure 5). 

## 5.1.2. Multiplicity 

As discussed in § 1, dynamical interactions between the disk and stellar companion is one of the main mechanisms proposed to account for the opacity holes of transition disks. 

However, the fraction of transition disks that are in fact close binaries still remains to be established. If binaries play a dominant role in transition disks, one would expect a higher incidence of close binaries in transition disks that in non-transition disks. In order to test this hypothesis, we compare the distribution of binary separations of our sample of transition disk to that of non-transition disk (and disk-less PMS stars). We drew our sample of non-transition disks and disk-less PMS stars from the compilation of Ophiuchus binaries presented by Cieza et al. (2009). 

As shown by Figure 7, disk-less PMS stars tend to have companions at smaller separations than stars with regular, non-transition disks. According to a two-sided Kolmogoro–Smirnov (KS) test, there is only a 0.5% probability that the distributions of binary separations of non-transition disks and disk-less stars have been drawn from the same parent population. As discussed by Cieza et al. (2009), this result can be understood in terms of the effect binaries have on circumstellar disks lifetimes via the tidal truncation 

