Carbon stars associated with oxygen-rich circumstellar envelopes : EU Andromedae, BM Geminorum and V 778 Cygni.

High signal-to-noise optical spectra are presented of three carbon stars associated with silicate emission, as well as an OH measurement (1665 and 1667 MHz) toward V 778 Cyg and upper limits for the CO (1-0) and (2-1) lines toward EU And and BM Gem. The optical spectra show that EU And is a J-type carbon star, as was previously known for BM Gem and V 778 Cyg. There is evidence that BM Gem and EU And show Tc-99, an s-process element which is considered to be a signature of recent dredge-up. Precise radial velocity measurements were performed for all three stars at a number of epochs. The two having Tc are also the only ones to show evidence of radial velocity variations. These variations are consistent with motion in a binary system. A binary model for these objects is considered in which the oxygen-rich dust ejected before the star became carbon-rich has been trapped in the system by the gravitational influence of a companion. The scenario does not require that both members of the binary be on the AGB.