  We present the results obtained with the CoRoT satellite for HD 50870, a
Delta Sct star which was observed for 114.4 d. The 307,570 CoRoT datapoints
were analysed with different techniques. The photometric observations were
complemented over 15 nights of high-resolution spectroscopy with HARPS on a
baseline of 25 d. Some uvby photometric observations were also obtained to
better characterize the pulsation modes. HD 50870 proved to be a low-amplitude,
long-period spectroscopic binary system seen almost pole-on (i~21 deg. The
brighter component, which also has the higher rotational velocity (v sin i=37.5
km/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 an
amplitude of about 0.045 mmag) a flat plateau appears in the power spectrum in
the low-frequency region up to about 35 c/d. We were able to detect this
plateau only thanks to the short cadence sampling of the CoRoT measurements (32
s). The density distribution vs. frequency of the detected frequencies seems
rule out the possibility that this plateau is the result of a process with a
continuum power spectrum. The spacings of the strongest modes suggest a
quasi-periodic pattern. We failed to find a satisfactory seismic model that
simultaneously matches the frequency range, the position in the HR diagram, and
the quasi-periodic pattern interpreted as a large separation. Nineteen modes
were detected spectroscopically from the line profile variations and associated
to the photometric ones. Tentative l,m values have been attributed to the modes
detected spectroscopically. Prograde as well as retrograde modes are present
with l degree values up to 9. There are no traces of variability induced by
solar-like oscillations.
