The interpolated free group factors L(F-r) for 1 < r less than or equa
l to infinity (also defined by F. Radulescu) are given another (but eq
uivalent) definition as well as proofs of their properties with respec
t to compression by projections and free products. In order to prove t
he addition formula for free products, algebraic techniques are develo
ped which allow us to show R R congruent to L(F-2) where R is the hy
perfinite II1-factor.