We have studied electromagnetic wave instabilities fed by coexisting newborn (cometary) hydrogen and oxygen ions, illustrating the modifications encountered in the unstable dispersion topology as the relative orientation of the interplanetary magnetic field and the solar wind velocity is varied, discussing the free energy sources, and assessing the influence of the newborn particle density and temperature. Utilization of generalized Brillouin diagrams clarifies the physical nature of new left-hand polarized, costreaming modes excited by the newborn heavy ions and nonoscillatory, purely growing structures (unrelated to the mirror wave) generated by the two newborn ions species. Although the newborn protons stimulate resonant instabilities more efficiently in ion-rich environments, the hierarchy of wave growth changes as the ion densities decrease (increasing distance from the comet). Larger growth rates for modes associated with the heavy-ion beam may then occur. The interaction between the two coexisting ions is generally weak: each beam excites resonant instabilities without undue influence from the other newborn ion species in most dispersion domains. Cometary environments suggest the parameters of the adopted model, but the results are helpful in the interpretation of other observations of low-frequency wave activity in space. ¿ American Geophysical Union 1988