– 18 – 4.3. Accretion Rates limits) for our sample, derived adopting a distance to Ophiuchus of 125 pc (Loinard et al. 2008), are listed in Table 3. For two objects, #12 and 27, we adopt the disk mass upper limits from Cieza et al. (2008), which were derived from 850 µ m observations. The vast majority of our transition disks have estimated disk masses lower than ∼ 2.0 M$_{JUP}$ . However, two of them have disk masses typical of CTTSs ( ∼ 3-5 M$_{JUP}$ ) and two others have significantly more massive disks ( ∼ 10-40 M$_{JUP}$ ). Most PMS stars show H α emission, either from chromospheric activity or accretion. Non-accreting objects show narrow ( < 200 km/s) and symmetric line profiles of chromspheric origin, while accreting objects present broad ( > 300 km/s) and asymmetric profiles produced by large-velocity magnetospheric accretion columns. The velocity dispersions (at 10 % of the peak intensity), Δ V , of the H α emission lines of our transition disks are listed in Table 2. The boundary between accreting and non-accreting objects has been empirically placed by different studies at Δ V between 200 km/s (Jayawardana et al. 2003) and 270 km/s (White & Basri, 2003). Since only one of our objects, source # 25, has Δ V in the 200-300 km/s range, most accreting and non-accreting objects are clearly separated in our sample. Source # 25 has Δ V ∼ 230 km/s and a very noisy spectrum. We consider it to be non-accreting because it has a very low fractional disk luminosity (L$_{D}$/L$_{∗}$ < 10 − $^{3}$, see § 5.1.3). The continuum-subtracted H α profiles for all the 17 accreting transition disks are shown in Figure 2, while those for the 8 non-accreting disks where the H α line was detected are shown in Figure 3. For accreting objects, the velocity dispersion of the H α line correlates well with accretion rates derived from detailed models of the magnetospheric accretion process. We therefore estimate the accretion rates of our targets from the width of the