  We investigate accreting disk systems with polytropic gas in Keplerian
motion. Numerical data and partial analytic results show that the
self-gravitation of the disk speeds up its rotation -- its rotational frequency
is larger than that given by the well known strictly Keplerian formula that
takes into account the central mass only. Thus determination of central mass in
systems with massive disks requires great care -- the strictly Keplerian
formula yields only an upper bound. The effect of self-gravity depends on
geometric aspects of disk configurations. Disk systems with a small (circa
$10^{-4}$) ratio of the innermost radius to the outermost disk radius have the
central mass close to the upper limit, but if this ratio is of the order of
unity then the central mass can be smaller by many orders of magnitude from
this bound.
