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Superconductivity and ferromagnetic ordering are two antagonistic types of ordering, and their mutual influence leads to many interesting phenomena which have been studied recently in ternary compounds. Theoretical analysis of ferromagnetic materials which are type II superconductors near the superconducting transition point T _cl shows that they become type I near the magnetic transition point T _M. The proposed theory constructed for the case T _M « T _cl predicts the formation of a transverse domain-like (DS phase) magnetic structure below T _M. The electronic spectrum appears to be gapless in the DS phase of clean compounds with a re-entrant transition. The change from type II to type I behaviour as the sample is cooled to T _M has been observed in ErRh₄B₄. Experimental data for HoMo₆S₈, HoMo₆Se₈ and ErRh₄B₄ give evidence for the coexistence of super-conductivity and non-uniform magnetic …