  Unveiling the fate of ultra-short period (USP) planets may help us understand
the qualitative agreement between tidal theory and the observed exoplanet
distribution. Nevertheless, due to the time-varying interchange of spin-orbit
angular momentum in star-planet systems, the expected amount of tidal friction
is unknown and depends on the dissipative properties of stellar and planetary
interiors. In this work, we couple structural changes in the star and the
planet resulting from the energy released per tidal cycle and simulate the
orbital evolution of USP planets and the spin-up produced on their host star.
For the first time, we allow the strength of magnetic braking to vary within a
model that includes photo-evaporation, drag caused by the stellar wind, stellar
mass loss, and stellar wind enhancement due to the in-falling USP planet. We
apply our model to the two exoplanets with the shortest periods known to date,
NGTS-10b and WASP-19b. We predict they will undergo orbital decay in
time-scales that depend on the evolution of the tidal dissipation reservoir
inside the star, as well as the contribution of the stellar convective envelope
to the transfer of angular momentum. Contrary to previous work, which predicted
mid-transit time shifts of $\sim30-190$ s over 10 years, we found that such
changes would be smaller than 10 s. We note this is sensitive to the
assumptions about the dissipative properties of the system. Our results have
important implications for the search for observational evidence of orbital
decay in USP planets, using present and future observational campaigns.
