  We show that in cubic crystals with anisotropic impurity centers the sum of
squares of the magnetic resonance (EPR) frequencies is invariant with respect
to the magnetic field direction. The connection between such an invariant and
the g-tensor components of the impurity is derived for different types of
centers. The established regularity is confirmed experimentally for the
spin-noise spectra of a CaF2-Nd3+ crystal. We show how this property of the EPR
spectra can be efficiently used for the assignment of paramagnetic centers in
cubic crystals.
