0. Introduction. Let X be a non-singular, connected, projective curve defined over an algebraically closed field k. Let U denote the set of isomorphism classes of stable vector bundles on X with given degree d and rank r. In the sixties, C.S. Seshadri and D. Mumford ([14], [17] and [18]) supplied U with a natural structure of quasi-projective variety, together with a natural compactification U ...