Calculated spectra for HeH+ and its effect on the opacity of cool metal-poor stars

The wavelength and Einstein A coefficient are calculated for all rotation-vibration transitions of (HeH+)-He-4-H-1, (HeH+)-He-3-H-1, (HeH+)-He-4-H-2 and (HeH+)-He-3-H-2, giving a complete line list and the partition function for (HeH+)-He-4 and its isotopologues. This opacity is included in the calculation of the total opacity of low-metallicity stars and its effect is analysed for different conditions of temperature, density and hydrogen number fraction. For a low helium number fraction (as in the Sun), it is found that HeH+ has a visible but small effect for very low densities (rho less than or equal to 10(-1)0 g cm(-3)), at temperatures around 3500 K. However, for high helium number fraction, the effect of HeH+ becomes important for higher densities (rho less than or equal to 10(-6) g cm(-3)), its effect being most important for a temperature around 3500 K. Synthetic spectra for a variety of different conditions are presented.