  The V1298 Tau system (20-30Myr), is a benchmark young multi-planet system
that provides the opportunity to perform comparative exoplanetology between
planets orbiting the same star right after their formation.
  We present the first atmospheric comparison between two planets in the same
transiting system: V1298 Tau b and V1298 Tau c. We derive constraints on the
mass of planet b and c (<20M$_\oplus$ at 3$\sigma$ confidence level and
$17_{-6}^{+13} M_{\oplus}$ respectively) and atmospheric metallicity
(logZ/Z$_\odot$=-2.04$_{-0.59}^{0.69}$, -0.16$_{-0.94}^{1.15}$ respectively)
from atmospheric retrievals. The V1298 Tau planets, are likely to be similar in
terms of mass at the current age, implying that both planets are potential
sub-Neptune/super-Earth progenitors. However, planet c is expected to lose a
higher fraction of its mass compared to planet b given its close proximity to
the host star. Alternatively, the observed spectrum of planet c can be
explained by atmospheric hazes, which is in contrast to planet b where
efficient haze formation can be ruled out. Higher haze formation efficiency in
planet c could be due to differences in atmospheric composition, temperature
and higher UV flux incident compared to planet b.
