  An indivisible object may be sold to one of $n$ agents who know their
valuations of the object. The seller would like to use a revenue-maximizing
mechanism but her knowledge of the valuations' distribution is scarce: she
knows only the means (which may be different) and an upper bound for
valuations. Valuations may be correlated.
  Using a constructive approach based on duality, we prove that a mechanism
that maximizes the worst-case expected revenue among all deterministic
dominant-strategy incentive compatible, ex post individually rational
mechanisms is such that the object should be awarded to the agent with the
highest linear score provided it is nonnegative. Linear scores are
bidder-specific linear functions of bids. The set of optimal mechanisms
includes other mechanisms but all those have to be close to the optimal linear
score auction in a certain sense. When means are high, all optimal mechanisms
share the linearity property. Second-price auction without a reserve is an
optimal mechanism when the number of symmetric bidders is sufficiently high.
