  X-rays emitted by the coronae of solar-type stars are a feature present in up
to late-A types during the main sequence phase. F stars, either with or without
hot Jupiters, are usually X-ray emitters. The very low level of X-ray emission
of the F5 star WASP-18 despite its relatively young age and spectral type is
thus quite peculiar. [Abridged] We observed KELT-24 with \xmm\ for a total of
43 ks in order to test if the X-ray activity of this star is depressed by the
interaction with its massive hot Jupiter, as is the case of WASP-18. KELT-24 is
detected in combined EPIC images with a high significance level. Its average
coronal spectrum is well described by a cool component at 0.36 keV and a hotter
component at 0.98 keV. We detected a flare with a duration of about 2 ks,
during which the coronal temperature reached 3.5 keV. The unabsorbed quiescent
flux in 0.3-8.0 keV is $\sim1.33\times10^{-13}$ erg s$^{-1}$ cm$^{-2}$,
corresponding to a luminosity of $1.5\times10^{29}$ erg s$^{-1}$ at the
distance of the star. The luminosity is well within the range of the typical
X-ray luminosity of F stars in Hyades, which are coeval. We conclude that the
activity of KELT-24 appears normal, as expected, and is not affected by any
star--planet interaction. From the analysis of TESS light curves, we infer a
distribution of optical flares for KELT-24 and WASP-18. Small optical
flickering similar to flares is recognized in WASP-18 but at lower levels of
energy and amplitude than in KELT-24. We discuss the causes of the low activity
of WASP-18. Either WASP-18b could hamper the formation of a corona bright in
X-rays in its host star through some form of tidal interaction, or the star has
entered a minimum of activity similar to the solar Maunder minimum. This latter
hypothesis would make WASP-18 among the few candidates showing such a quench of
stellar activity.
