  A well known solution of heterotic string theory is the spinning Kerr-Sen
black hole (KSBH) characterized by a string parameter $\xi$. Kerr black hole is
recovered at $\xi=0$. The purpose of this paper is to investigate the effect of
$\xi$ on a new diagnostic of relative time delay (RTD) to see how the latter
deviates from that in general relativity. Assuming KSBH as the spinning lens
partner in PSR-BH binary systems, which provide the best laboratory for testing
the time delay predictions, we study here the RTD up to third PPN order in
$\left(1/r\right)$ in the thin-lens approximation. We work out a useful
generalization of the RTD formulas applicable to the experimentally viable
\textit{finite} distance lens scales, while\ terms higher than the zeroth order
are shown to contain the effect of $\xi$. We shall also relate RTD to the
observable image magnification factor determined by $\beta/\theta_{E}$, where
$\beta$ is the angular separation between the source and the observer and
$\theta_{E}$ is the "Einstein angle" determined by an "effective" non-aligned
static lens equivalent to the original aligned spinning lens. Numerical
estimates for two typical binary lens systems show $\mu$sec level delay at the
zeroth order consistent with predictions in the literature. However, the string
effect at higher orders is too tiny to be measurable even in the far future
leading to the conclusion that the stringy and general relativity BHs are yet
observationally indistinguishable.
