  We costruct the relativistic operator of the quadrupole moment of
two-particle composite spin one systems with zero orbital moment of the
relative motion and derive explicit analytical expression for the quadrupole
moment using the approach to relativistic composite systems based on our
version of the instant-form relativistic quantum mechanics (RQM). We calculate
the quadrupole moments of the rho meson and of the S-wave deuteron without any
free parameters, using our unified pi&rho model (Phys. Rev.D 93, 036007 (2016);
97, 033007 (2018)) and our previous results on deuteron. Our calculation gives
Q_rho=-0.158+-0.04 GeV^-2 and Q_d=-1.4*10^-4 GeV^-2. Having in our disposition
the rather general form of the quadrupole-moment operator we for the first time
formulate the problem of the upper and lower bounds for possible values of the
quadrupole moment of a two-particle system with indicated quantum numbers for a
large range of constituent masses, and partially solve it.
