The interpolated free group
factors L(Fr) for 1 < r ≤∞ (also defined by F. Rădulescu) 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.