Context : We present the transit and follow-up of a single transit event from Campaign 14 of K2 , EPIC248847494b , which has a duration of 54 hours and a 0.18 % depth .

Aims : Using photometric tools and conducting radial velocity follow-up , we vet and characterise this very strong candidate .

Methods : Owing to the long , unknown period , standard follow-up methods needed to be adapted .
The transit was fitted using Namaste , and the radial velocity slope was measured and compared to a grid of planet-like orbits with varying masses and periods .
These used stellar parameters measured from spectra and the distance as measured by Gaia .

Results : Orbiting around a sub-giant star with a radius of 2.70 \pm 0.12R _ { Sol } , the planet has a radius of 1.11 _ { -0.07 } ^ { +0.07 } R _ { Jup } and a period of 3650 _ { -1130 } ^ { +1280 } days .
The radial velocity measurements constrain the mass to be lower than 13M _ { Jup } , which implies a planet-like object .

Conclusions : We have found a planet at 4.5 AU from a single-transit event .
After a full radial velocity follow-up campaign , if confirmed , it will be the longest-period transiting planet discovered .