Context :

Aims : We study a sample composed of 28 of the brightest stars in the Arches cluster .
Our aim is to constrain their stellar and wind properties and to establish their nature and evolutionary status .

Methods : We analyze K-band spectra obtained with the integral field spectrograph SINFONI on the VLT .
Atmosphere models computed with the code CMFGEN are used to derive the effective temperatures , luminosities , stellar abundances , mass loss rates and wind terminal velocities .

Results : We find that the stars in our sample are either H-rich WN7–9 stars ( WN7–9h ) or supergiants , two being classified as OIf ^ { + } .
All stars are 2–4 Myr old .
There is marginal evidence for a younger age among the most massive stars .
The WN7–9h stars reach luminosities as large as 2 \times 10 ^ { 6 } L _ { \odot } , consistent with initial masses of \sim 120 M _ { \odot } .
They are still quite H-rich , but show both N enhancement and C depletion .
They are thus identified as core H-burning objects showing products of the CNO equilibrium at their surface .
Their progenitors are most likely supergiants of spectral types earlier than O4–6 and initial masses > 60 M _ { \odot } .
Their winds follow a well defined modified wind momentum – luminosity relation ( WLR ) : this is a strong indication that they are radiatively driven .
Stellar abundances tend to favor a slightly super solar metallicity , at least for the lightest metals .
We note however that the evolutionary models seem to under-predict the degree of N enrichment .

Conclusions :