Fluctuations of ''instantaneous'' virial masses of stationary gravitating systems

The use of the values of the kinetic and potential energy for an arbitrary moment of time instead of the time-averaged values required by the virial theorem leads to an estimate of the mass of the system Msub(VT)(t) not coinciding with the real mass M. For closed binary systems, the virial ratio V=Msub(VT)/M lies within the range [0-2]. For a system of N>> 1 bodies the standard deviation σ(V) approximately equal to Nsup(-1/2). The difference of Msub(VT) from M for a given pair or group of galaxies should not be considered as an indication to the instability of the system. The conclusions on the stability must refer to the whole totality of systems