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Context. Observations and analysis of solar-type oscillations in red-giantstars is an emerging aspect of asteroseismic analysis with a number of openquestions yet to be explored. Although stochastic oscillations have previouslybeen detected in red giants from both radial velocity and photometricmeasurements, those data were either too short or had sampling that was notcomplete enough to perform a detailed data analysis of the variability. Thequality and quantity of photometric data as provided by the CoRoT satellite isnecessary to provide a breakthrough in observing p-mode oscillations in redgiants. We have analyzed continuous photometric time-series of about 11 400relatively faint stars obtained in the exofield of CoRoT during the first 150days long-run campaign from May to October 2007. We find several hundred starsshowing a clear power excess in a frequency and amplitude range expected forred-giant pulsators. In this paper we present first results on a sub-sample ofthese stars. Aims. Knowing reliable fundamental parameters like mass and radiusis essential for detailed asteroseismic studies of red-giant stars. As theCoRoT exofield targets are relatively faint (11-16 mag) there are no (or onlyweak) constraints on the star's location in the H-R diagram. We therefore aimto extract information about such fundamental parameters solely from theavailable time series. Methods. We model the convective background noise andthe power excess hump due to pulsation with a global model fit and deducereliable estimates for the stellar mass and radius from scaling relations forthe frequency of maximum oscillation power and the characteristic frequencyseparation.

Oscillating red giants in the CoRoT exo-field: Asteroseismic mass and radius determination