uvby photometry of very hot stars in the Large Magellanic Cloud.

Summary. uvby photometry has been carried out on 53 of the hottest stars in the LMC, selected on the basis of their rough spectral classifications and their UB V colours. The c1, b - y diagram con firms that many of them have Teff >= 30000 0K, according to NLTE models of Mihalas. E(b - y) excesses are derived, giving a mean value of the visual absorption Av =0.37. The absolute magnitudes of the presumed 0 stars (Teff >30000 0K) are found to be quite consistent with the calibration by Conti for galactic 0 stars. The Mbol - log Teff diagram is established and it extends well Osmer's one on the hot side. The likeness of the left part of this diagram with the diagram of galactic O stars is clearly evident when Hcc emission stars are excepted, giving an upper mass limit around 90 M0 for stars with Teff <40000 0K, and reaching 120 M0 for the hottest stars. Moreover, the plot of B - V versus b - y colours for stars in common shows a reasonable agreement with Crawford's relation, and the (U - B)0 - (B - V)0 diagram deduced from the E(b - y) excesses agrees correctly with the intrinsic sequences of supergiants. Key words: uvby photometry 0 stars Large Magellanic Cloud