  The Transiting Exoplanet Survey Satellite (TESS) is observing bright known
planet-host stars across almost the entire sky. These stars have been subject
to extensive ground-based observations, providing a large number of radial
velocity (RV) measurements. In this work we use the new TESS photometric
observations to characterize the star $\lambda^2$ Fornacis, and following this
to update the parameters of the orbiting planet $\lambda^2$ For b. We measure
the p-mode oscillation frequencies in $\lambda^2$ For, and in combination with
non-seismic parameters estimate the stellar fundamental properties using
stellar models. Using the revised stellar properties and a time series of
archival RV data from the UCLES, HIRES and HARPS instruments spanning almost 20
years, we refit the orbit of $\lambda^2$ For b and search the RV residuals for
remaining variability. We find that $\lambda^2$ For has a mass of
$1.16\pm0.03$M$_\odot$ and a radius of $1.63\pm0.04$R$_\odot$, with an age of
$6.3\pm0.9$Gyr. This and the updated RV measurements suggest a mass of
$\lambda^2$ For b of $16.8^{+1.2}_{-1.3}$M$_\oplus$, which is $\sim5$M$_\oplus$
less than literature estimates. We also detect a periodicity at 33 days in the
RV measurements, which is likely due to the rotation of the host star. While
previous literature estimates of the properties of $\lambda^2$ are ambiguous,
the asteroseismic measurements place the star firmly at the early stage of its
subgiant evolutionary phase. Typically only short time series of photometric
data are available from TESS, but by using asteroseismology it is still
possible to provide tight constraints on the properties of bright stars that
until now have only been observed from the ground. This prompts a reexamination
of archival RV data from the past few decades to update the characteristics of
the planet hosting systems observed by TESS for which asteroseismology is
possible.
