Evidence for a population of numerous binaries with comparably bright components among Hipparcos "single" F stars

We find evidence for the presence of a population of numerous binaries with comparably bright components (C binaries) among Hipparcos F stars that are flagged in the Hipparcos catalog as single. We propose a criterion that id entifies such binaries. The criterion is based on comparison of absolute ma gnitude M-v obtained from the Hipparcos parallax and Johnson V magnitude wi th absolute magnitude derived from the dereddened uvby luminosity index c(o ), M-co. For photometrically unresolved binaries, M-v must be brighter beca use it represents the integrated flux of a binary system, while M-co is det ermined by the integrated spectrum and is thus close to the absolute magnit ude of the primary. We have conducted a series of tests that have revealed the presence of C binaries among Hipparcos "single" F stars and confirmed t hat the proposed criterion is quite effective at identifying these stars. T he C binary candidates have been found to be, on average, older than the ca ndidates comprising single stars and binaries with disparate components (D binaries).