In an incomplete market framework, contingent claims are of particular
interest since they improve the market efficiency. This paper address
es the problem of market completeness when trading in contingent claim
s is allowed. We extend recent results by Bajeux and Rochet (1996) in
a stochastic volatility model to the case where the asset price and it
s volatility variations are correlated. We also relate the ability of
a given contingent claim to complete the market to the convexity of it
s price function in the current asset price. This allows us to state o
ur results for general contingent claims by examining the convexity of
their ''admissible arbitrage prices.''.