Bound states near the lower continuum boundary

The effective range approximation is extended to the case of the Dirac equation. Exact formulas are obtained which express the parameters of the expansion of S-matrix at epsilon→-mc2 in terms of the wave function of the level at critical point. With the aid of the formulas obtained the embedding of discrete spectrum levels into the lower continuum is investigated. Critical charge of the nucleus for muons is evaluated (the case R>>h/mc)