Context :

Aims : We present the results obtained with the CoRoT satellite for HD 50870 , a \delta Sct star which was observed for 114.4 d. The aim of these observations was to evaluate the results obtained for HD 50844 , the first \delta Sct star monitored with CoRoT , on a longer time baseline .

Methods : 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. These spectra were analysed to study the line profile variations and to derive the stellar physical parameters .
Some uvby photometric observations were also obtained to better characterize the pulsation modes .

Results : HD 50870 proved to be a low-amplitude , long-period spectroscopic binary system seen almost pole-on ( i \simeq 21 ^ { o } ) .
The brighter component , which also has the higher rotational velocity ( v \sin i = 37.5 km s ^ { -1 } ) , is a \delta Sct-type variable with a full light amplitude variation of about 0.04 mag .
There is a dominant axisymmetric mode ( 17.16 d ^ { -1 } ) .
Moreover , there are two groups of frequencies ( about 19 ) in the intervals 6-9 and 13-18 d ^ { -1 } , with amplitudes ranging from a few mmag to 0.3 mmag .
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 d ^ { -1 } .
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 \ell,m values have been attributed to the modes detected spectroscopically .
Prograde as well as retrograde modes are present with \ell values up to 9 .
There are no traces of variability induced by solar-like oscillations .

Conclusions :