  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+-1000 K and log g = 3.45+-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+-0.0016 is obtained. Three modes are detected in photometry. The degree
l is unambiguously identified for two of them: l = 0 and l = 3 for the
frequencies 5.48694 1/d and 0.30818 1/d, 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 1/d found in photometry, two possibilities remain: l = 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 1/d, is
identified as (l,m)=(3,2). Based on this mode identification, we finally deduce
an equatorial rotational velocity of 38+-15 km/s.
