dispersion data, available in the literature, for a chosen galactocentric distance $R=2h$, where $h$ is the radial disk scalelenght. At this radius a disk contribution to the rotation curve is nearly maximal. At higher $R$, measurements of velocity dispersion are less reliable and also the influence of external factors dynamically heating a disk may be more important. In turn, at lower $R$ it is often hard to separate the contributions of bulge and disk stars to the measured values of $c_{obs}$, and to take into account the presence of a bar if it occurs in a galaxy. It is worth noting that, as numerical 3D N-body simulations show for different parameters of components (disk, bulge and halo), the spread of values of $Q(R)$ is minimal at $R\approx2h$. Following Khoperskov et al. (2003) here we assume $Q(2h)\approx 1.5$.

[**2. The sample and the method we use**]{}

To estimate the upper limits of disk masses corresponding to the marginal stability condition we have chosen objects for which sufficiently extensive radial distributions of stellar velocity dispersion and rotation curves were available in the literature (121 objects). Using these data we estimated the threshold value of surface density corresponding to the gravitational stability at 