  A short observing run with the spectrometer Harps at the ESO 3.6-m telescope
was conducted in order to continue exploring the asteroseismic properties of F
type stars. In fact, Doppler observations of F type on the main sequence are
demanding and remain currently limited to a single case (HD 49933). Comparison
with photometric results obtained with the CoRoT mission on similar stars will
be possible with an enhanced set of observations. We selected the 4th magnitude
F8V star HD 203608, in order to investigate the oscillating properties of a
low-metallicity star of the old galactic disk. A 5-night asteroseismic
observation program has been conducted in August 2006 with Harps. Spectra were
reduced with the on-line data reduction software provided by the instrument. A
new statistical approach has been developed for extracting the significant
peaks in the Fourier domain. The oscillation spectrum shows a significant
excess power in the frequency range [1.5, 3.0 mHz]. It exhibits a large spacing
about 120.4 $\mu$Hz at 2.5 mHz. Variations of the large spacing with frequency
are clearly identified, which require an adapted asymptotic development. The
modes identification is based on the unambiguous signature of 15 modes with
$\ell = 0$ and 1. This observation shows the potential diagnostic of
asteroseismic constraints. Including them in the stellar modeling enhances
significantly the precision on the physical parameters of \cible, resulting in
a much more precise position in the HR diagram. The age of the star is now
determined in the range $7.25\pm0.07$ Gyr.
