Context : The known \beta Cephei star HD 180642 was observed by the CoRoT satellite in 2007 .
From the very high-precision light curve , its pulsation frequency spectrum could be derived for the first time ( Degroote and collaborators ) .
In this paper , we obtain additional constraints for forthcoming asteroseismic modeling of the target .
Our results are based on both extensive ground-based multicolour photometry and high-resolution spectroscopy .
We determine T _ { eff } = 24 500 \pm 1000 K and \log g = 3.45 \pm 0.15 dex from spectroscopy .
The derived chemical abundances are consistent with those for B stars in the solar neighbourhood , except for a mild nitrogen excess .
A metallicity Z = 0.0099 \pm 0.0016 is obtained .
Three modes are detected in photometry .
The degree \ell is unambiguously identified for two of them : \ell = 0 and \ell = 3 for the frequencies 5.48694 d ^ { -1 } and 0.30818 d ^ { -1 } , respectively .
The radial mode is non-linear and highly dominant with an amplitude in the U-filter about 15 times larger than the strongest of the other modes .
For the third frequency of 7.36673 d ^ { -1 } found in photometry , two possibilities remain : \ell = 0 or 3 .
In the radial velocities , the dominant radial mode presents a so-called stillstand but no clear evidence of the existence of shocks is observed .
Four low-amplitude modes are found in spectroscopy and one of them , with frequency 8.4079 d ^ { -1 } , is identified as ( \ell,m ) = ( 3 , 2 ) .
Based on this mode identification , we finally deduce an equatorial rotational velocity of 38 \pm 15 km s ^ { -1 } .

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