This is far from a definitive answer, but I haven't heard of YCombinator investing in any Series A rounds. Dilution is part of the game, and is generally a good thing. Further investment generally boosts the value of a company making YC and any previous investors' investments already a win... on paper at least.

For a list of most, if not all, the companies YC has invested in take a look here: http://www.crunchbase.com/company/y-combinator and see if you can find any that aren't listed as "Angel" investments.