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Transit observations in the helium triplet around 10830 Angstrom are asuccessful tool to study exoplanetary atmospheres and their mass loss. Formingthose lines requires ionisation and recombination of helium in the exoplanetaryatmosphere. This ionisation is caused by stellar photons at extremeultra-violet (EUV) wavelengths; however, no currently active telescopes canobserve this part of the stellar spectrum. The relevant part of the stellar EUVspectrum consists of individual emission lines, many of them being formed byiron at coronal temperatures. The stellar iron abundance in the corona is oftenobserved to be depleted for high-activity low-mass stars due to the firstionisation potential (FIP) effect. I show that stars with high versus lowcoronal iron abundances follow different scaling laws that tie together theirX-ray emission and the narrow-band EUV flux that causes helium ionisation. Ialso show that the stellar iron to oxygen abundance ratio in the corona can bemeasured reasonably well from X-ray CCD spectra, yielding similar results tohigh-resolution X-ray observations. Taking coronal iron abundance into account,the currently observed large scatter in the relationship of EUV irradiationwith exoplanetary helium transit depths can be reduced, improving the targetselection criteria for exoplanet transmission spectroscopy. In particular,previously puzzling non-detections of helium for Neptunic exoplanets are now inline with expectations from the revised scaling laws.

Helium absorption in exoplanet atmospheres is connected to stellar coronal abundances