Self-bound CFL stars in binary systems : are they "hidden" among the black hole candidates?

The identification of black holes is one of the most important tasks of modern astrophysics. Candidates have been selected among binary stars based on a high mass function, and seriously considered when the lower mass limit exceeds ~ 3 solar masses. More recently the absence of (Type I) thermonuclear bursts has been advanced as an additional criterion in favor of the black hole interpretation, since the absence of a solid surface naturally precludes the accumulation and ignition of accreting material. We discuss in this Letter the possibility that self-bound stars made of CFL-paired quarks mimic the behavior of at least the low-mass end black holes as a result of a) higher maximum masses than ordinary neutron stars b) low steady luminosities due to the bare surface properties c) impossibility of generating Type I bursts because of the complete absence of normal matter crusts at their surfaces. These features caution about a positive identification of event horizons based on the lack of bursts.