  A single unit of a good is sold to one of two bidders. Each bidder has either
a high prior valuation or a low prior valuation for the good. Their prior
valuations are independently and identically distributed. Each bidder may
observe an independently and identically distributed signal about her prior
valuation. The seller knows the distribution of the prior valuation profile and
knows that signals are independently and identically distributed, but does not
know the signal distribution. In addition, the seller knows that bidders play
undominated strategies. I find that a second-price auction with a random
reserve maximizes the worst-case expected revenue over all possible signal
distributions and all equilibria in undominated strategies.
