K2-146 is a cool , 0.358 M _ { \odot } dwarf that was found to host a mini-Neptune with a 2.67-days period .
The planet exhibited strong transit timing variations ( TTVs ) of greater than 30 minutes , indicative of the presence of a further object in the system .
Here we report the discovery of the previously undetected outer planet , K2-146 c , in the system using additional photometric data .
K2-146 c was found to have a grazing transit geometry and a 3.97-day period .
The outer planet was only significantly detected in the latter K2 campaigns presumably because of precession of its orbital plane .
The TTVs of K2-146 b and c were measured using observations spanning a baseline of almost 1200 days .
We found strong anti-correlation in the TTVs , suggesting the two planets are gravitationally interacting .
Our TTV and transit model analyses revealed that K2-146 b has a radius of 2.25 \pm 0.10 R _ { \oplus } and a mass of 5.6 \pm 0.7 M _ { \oplus } , whereas K2-146 c has a radius of 2.59 _ { -0.39 } ^ { +1.81 } R _ { \oplus } and a mass of 7.1 \pm 0.9 M _ { \oplus } .
The inner and outer planets likely have moderate eccentricities of e = 0.14 \pm 0.07 and 0.16 \pm 0.07 , respectively .
Long-term numerical integrations of the two-planet orbital solution show that it can be dynamically stable for at least 2 Myr .
The evaluation of the resonance angles of the planet pair indicates that K2-146 b and c are likely trapped in a 3:2 mean motion resonance .
The orbital architecture of the system points to a possible convergent migration origin .