On the unidentified bands 6830, 7088 in symbiotic stars

About 60 stars are known which show broad emission bands centred at wavelengths of 6830 and 7088 Å. The stars are all classified as symbiotic, since they combine high-excitation emission and M-type absorption spectra. From the behaviour of the bands in the evolution of slow novae as they approach the symbiotic phase, and from the occurrence of the bands in stars of different excitation, it is concluded that the ions responsible have ionization potentials near 100eV. The similarity of behaviour and profile of the two suggests that both arise in the same species. No suitable identification appears possible at this time, because of the lack of data on highly ionized species. Arguments are presented which narrow the range of possibilities, the most notable argument being the absence of O VI emission. It is suggested that Fe VII or Fe VI may be responsible. In particular, it is recommended that transitions from the $$\text{Z}^{3}\text{P}^{0}\,\text{and}\,\text{Z}^{1}\text{F}^{0}$$ levels of Fe VII be examined in detail. The differing, and time-varying profiles of the 6830 and 7088 bands in the stars observed are best explained in terms of velocity broadening. Velocities in excess of 1000 km s^–1 are present. Rotation is a more credible form of the mass motion than expansion, because of the tendency to double profiles in these bands. If rotation is responsible, these velocities imply that the objects central to the emission nebulae are more compact than main sequence stars. Within each star, the similarity in profile of the two bands militates against the currently fashionable single-star model for slow novae, in which TiO absorption must overlie the band emission region, and should selectively absorb parts of the 7088 band.