  We give precise conditions under which the real interpolation space
[Y_0,X_1]_{s,p} coincides with a closed subspace of the corresponding
interpolation space [X_0,X_1]_{s,p} when Y_0 is a closed subspace of X_0 of
codimension one. This result is applied to study the basis properties of
nonharmonic Fourier series in Sobolev spaces H^s on an interval when 0<s<1. The
main result: let E be a family of exponentials exp(i \lambda_n t) and E forms
an unconditional basis in L^2 on an interval. Then there exist two number s_0,
s_1 such that E forms an unconditional basis in H^s for s<s_0, E forms an
unconditional basis in its span with codimension 1 in H^s for s_1<s. For s in
[s_0,s_1] the exponential family is not an unconditional basis in its span.
