Catalyzes the removal of O-acetyl ester groups from position 9 of the parent sialic acid, N-acetylneuraminic acid. H2O + N-acetyl-9-O-acetylneuraminate = acetate + H(+) + N-acetylneuraminate H2O + N-acetyl-4-O-acetylneuraminate = acetate + H(+) + N-acetylneuraminate