  Transit observations in the helium triplet around 10830 Angstrom are a
successful tool to study exoplanetary atmospheres and their mass loss. Forming
those lines requires ionisation and recombination of helium in the exoplanetary
atmosphere. This ionisation is caused by stellar photons at extreme
ultra-violet (EUV) wavelengths; however, no currently active telescopes can
observe this part of the stellar spectrum. The relevant part of the stellar EUV
spectrum consists of individual emission lines, many of them being formed by
iron at coronal temperatures. The stellar iron abundance in the corona is often
observed to be depleted for high-activity low-mass stars due to the first
ionisation potential (FIP) effect. I show that stars with high versus low
coronal iron abundances follow different scaling laws that tie together their
X-ray emission and the narrow-band EUV flux that causes helium ionisation. I
also show that the stellar iron to oxygen abundance ratio in the corona can be
measured reasonably well from X-ray CCD spectra, yielding similar results to
high-resolution X-ray observations. Taking coronal iron abundance into account,
the currently observed large scatter in the relationship of EUV irradiation
with exoplanetary helium transit depths can be reduced, improving the target
selection criteria for exoplanet transmission spectroscopy. In particular,
previously puzzling non-detections of helium for Neptunic exoplanets are now in
line with expectations from the revised scaling laws.
