The Incidence of Binaries among Very Metal-poor Carbon Stars

We have monitored the radial velocities of seven very metal-poor stars with abnormally strong CH G bands. On the basis of their colors four are giants (gCH) and three are subgiants (sgCH). Three of the gCH stars are definite spectroscopic binaries with long periods, 505 days < P < 3125 days. Two of these possess s -process overabundances, while the third does not. The binary status of the fourth, the r -process–rich giant CS 22892-052, is uncertain. The three sgCH stars possess marked s -process overabundances, but contrary to the expectations of the asymptotic giant branch mass transfer paradigm, their radial velocities have remained constant with σ ≃ ±0.4 km s -1 for the past eight years. Comparison of the standard deviations of our radial velocities with those of McClure's sgCH binaries indicates that the probability that our three stars belong to the population of sgCH binary stars is very small. The dereddened colors and effective temperatures of these stars are similar to those of LP 706-7, which has similar composition and also no measurable velocity variation. The carbon overabundances and the heavy-to-light s -process ratios of all four stars exceed those of disk sgCH stars by an order of magnitude. We return to Bond's 1974 suggestion that these are post–He flash stars that have mixed some envelope material into their cores according to a scheme proposed by Rood.