Three 0⁺ modes identified in the spectrum of Altair. The top row shows the surface amplitude of the relative pressure fluctuation associated with the mode as seen from the Earth. The bottom row shows a meridional cut of the same quantity along with a zoom of the subsurface layers, with the depth being limited to 5% of the equatorial radius. The solid line in the meridional cut of the star shows the trajectory of an acoustic ray associated with the mode. The dashed black line in the zoomed layers highlights the 50 000 K isotherm around which the second ionisation of helium takes place.