  Approximately 10% of massive OBA main-sequence (MS) and pre-MS stars harbour
strong, large-scale magnetic fields. At the same time there is a dearth of
magnetic stars in close binaries. A process generating strong magnetic fields
only in some stars must be responsible with the merging of pre-MS and MS stars
being suggested as one such channel. Stars emerging from the coalescence of two
MS stars are rejuvenated, appearing younger than they are. They can therefore
be identified by comparison with reference clocks. Here we predict the
rejuvenation of MS merger products over a wide range of masses and binary
configurations calibrated to smoothed-particle-hydrodynamical merger models. We
find that the rejuvenation is of the order of the nuclear timescale and is
strongest in the lowest-mass mergers and the most evolved binary progenitors
with the largest mass ratios. These predictions allow us to put constraints on
the binary progenitors of merger products. We show that the magnetic stars HR
2949 and $\tau$ Sco are younger than the potential binary companion HR 2948 and
the Upper-Sco association, respectively, making them promising merger
candidates. We find that the age discrepancies and the potential binary
progenitors of both are consistent with them being rejuvenated merger products,
implying that their magnetic fields may originate from this channel. Searching
for age discrepancies in magnetic stars is therefore a powerful way to explore
which fraction of magnetic stars may have obtained their strong magnetic field
in MS mergers and to improve our understanding of magnetism in massive stars
and their remnants.
