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We present the results obtained with the CoRoT satellite for HD 50870, aDelta Sct star which was observed for 114.4 d. The 307,570 CoRoT datapointswere analysed with different techniques. The photometric observations werecomplemented over 15 nights of high-resolution spectroscopy with HARPS on abaseline of 25 d. Some uvby photometric observations were also obtained tobetter characterize the pulsation modes. HD 50870 proved to be a low-amplitude,long-period spectroscopic binary system seen almost pole-on (i~21 deg. Thebrighter component, which also has the higher rotational velocity (v sin i=37.5km/s), is a delta Sct-type variable. There is a dominant axisymmetric mode(17.16 c/d). After the detection of about 250 terms (corresponding to anamplitude of about 0.045 mmag) a flat plateau appears in the power spectrum inthe low-frequency region up to about 35 c/d. We were able to detect thisplateau only thanks to the short cadence sampling of the CoRoT measurements (32s). The density distribution vs. frequency of the detected frequencies seemsrule out the possibility that this plateau is the result of a process with acontinuum power spectrum. The spacings of the strongest modes suggest aquasi-periodic pattern. We failed to find a satisfactory seismic model thatsimultaneously matches the frequency range, the position in the HR diagram, andthe quasi-periodic pattern interpreted as a large separation. Nineteen modeswere detected spectroscopically from the line profile variations and associatedto the photometric ones. Tentative l,m values have been attributed to the modesdetected spectroscopically. Prograde as well as retrograde modes are presentwith l degree values up to 9. There are no traces of variability induced bysolar-like oscillations.

Pulsation spectrum of Delta Sct stars: the binary HD 50870 as seen with CoRoT and HARPS