K 0 S , Λ, Ξ, Ω and negative particle yields and transverse mass spectra have been measured at central rapidity in Pb-Pb and p-Pb collisions at 158 A GeV/c. Yields are studied as a function of the number of nucleons participating in the collision N part , which is estimated with the Glauber model. From p-Pb to Pb-Pb collisions the particle yields per participant increase substantially. The enhancement is more pronounced for multistrange particles, and exceeds an order of magnitude for the Ω. For a number of participants, N part , greater than 100, however, all yields per participant appear to be constant. To be published in Physics Letters B

Pb interactions are presented as a function of the collision centrality and compared with those obtained from p-Pb collisions. Strangeness enhancement is observed which increases with centrality and with the strangeness content of the hyperon.

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We report the main results from experiment WA97 on the production of strange and multistrange hadrons at midrapidity in Pb+Pb, p+Pb and p+Be collisions at 158 A GeV/c. A comparison with some of the available models is presented. INTRODUCTIONThe WA97 experiment was designed to study the production in nucleus-nucleus collisions of hadrons carrying one, two and three units of strangeness as a function of the centrality of the collision. This paper describes the main results obtained so far. These results are then compared with some model predictions.The WA97 apparatus is briefly described in section 2. Section 3 deals with the measurement of the collision centrality. The results on the production of strange and multistrange hadrons are presented in sections 4 (yields) and 5 (m T slopes). WA97 has isolated a sample of double V 0 events; a first look at these data is offered in section 6. A thorough comparison with the available models is now possible. We present some of our own work in this field in section 7. Section 8 presents an outlook on the programme of the NA57 experiment, the successor of WA97, currently on the floor. We present our conclusions in section 9.