  The wavelength and Einstein A coefficient are calculated for all
rotation-vibration transitions of $^4$He$^1$H$^+$, $^3$He$^1$H$^+$,
$^4$He$^2$H$^+$ and $^3$He$^2$H$^+$, giving a complete line list and the
partition function for $^4$HeH$^+$ 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
($\rho\leq 10^{-10}$g cm$^{-3}$), at temperatures around 3500 K. However, for
high helium number fraction, the effect of HeH$^+$ becomes important for higher
densities ($\rho\leq 10^{-6}$g cm$^{-3}$), its effect being most important for
a temperature around 3500 K. Synthetic spectra for a variety of different
conditions are presented.
