  Mode identification is crucial for an asteroseismological study of any
significance. Contrarily to spectroscopic techniques, methods such as
period-fitting and multi-colour photometry do not provide a full reconstruction
of non-radial pulsations. We present a new method of spectroscopic mode
identification and test it on time-series of synthetic spectra appropriate for
pulsating subdwarf-B stars. We then apply it to the newly discovered slowly
pulsating subdwarf-B star HD 4539.
