  We report the discovery of two transiting planets orbiting K2-290 (EPIC
249624646), a bright (V=11.11) late F-type star residing in a triple-star
system. It was observed during Campaign 15 of the K2 mission, and in order to
confirm and characterise the system, follow-up spectroscopy and AO imaging were
carried out using the FIES, HARPS, HARPS-N, and IRCS instruments. From AO
imaging and Gaia data we identify two M-dwarf companions at a separation of
$113 \pm 2$ AU and $2467_{-155}^{+177}$ AU. From radial velocities, K2
photometry, and stellar characterisation of the host star, we find the inner
planet to be a mini-Neptune with a radius of $3.06 \pm 0.16 R_{\oplus}$ and an
orbital period of $P = 9.2$ days. The radius of the mini-Neptune suggests that
the planet is located above the radius valley, and with an incident flux of
$F\sim 400 F_{\oplus}$, it lies safely outside the super-Earth desert. The
outer warm Jupiter has a mass of $0.774\pm 0.047 M_{\rm J}$ and a radius of
$1.006\pm 0.050R_{\rm J}$, and orbits the host star every 48.4 days on an orbit
with an eccentricity $e<0.241$. Its mild eccentricity and mini-Neptune sibling
suggest that the warm Jupiter originates from in situ formation or disk
migration.
