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A short observing run with the spectrometer Harps at the ESO 3.6-m telescopewas conducted in order to continue exploring the asteroseismic properties of Ftype stars. In fact, Doppler observations of F type on the main sequence aredemanding and remain currently limited to a single case (HD 49933). Comparisonwith photometric results obtained with the CoRoT mission on similar stars willbe possible with an enhanced set of observations. We selected the 4th magnitudeF8V star HD 203608, in order to investigate the oscillating properties of alow-metallicity star of the old galactic disk. A 5-night asteroseismicobservation program has been conducted in August 2006 with Harps. Spectra werereduced with the on-line data reduction software provided by the instrument. Anew statistical approach has been developed for extracting the significantpeaks in the Fourier domain. The oscillation spectrum shows a significantexcess power in the frequency range [1.5, 3.0 mHz]. It exhibits a large spacingabout 120.4 $\mu$Hz at 2.5 mHz. Variations of the large spacing with frequencyare clearly identified, which require an adapted asymptotic development. Themodes identification is based on the unambiguous signature of 15 modes with$\ell = 0$ and 1. This observation shows the potential diagnostic ofasteroseismic constraints. Including them in the stellar modeling enhancessignificantly the precision on the physical parameters of \cible, resulting ina much more precise position in the HR diagram. The age of the star is nowdetermined in the range $7.25\pm0.07$ Gyr.

HD 203608, a quiet asteroseismic target in the old galactic disk