The interpolated free group factors L(Fr) for 1 < r (also defined by F. Radulescu) are given another (but equivalent) definition as well as proofs of their properties with respect to compression by projections and free products. In order to prove the addition formula for free products, algebraic techniques are developed which allow us to show R*R L(F2) where R is the hyperfinite II1 -factor. 1994 by Pacific Journal of Mathematics.
