  We prove the Oppenheim conjecture for indefinite ternary diagonal forms of
the type $x^{2}+y^{2} -\alpha z^{2}$ where $ \alpha $ is an irrational number.
Our method is explicit in the sense that we are able to construct a solution to
the problem and we obtain an effective bound on the solution. The method is
geometrical and is based on continued fractions.
