It is shown that the groups
F
/
[
F
n
,
F
m
]
F/[{F_n},{F_m}]
, where
F
F
is a free group and
m
,
n
m,n
are positive integers such that
m
>
n
≤
2
m
m > n \leq 2m
, are residually “torsion-free and nilpotent", and the structure of their lower central factors is computed.