  K2-146 is a cool, 0.358 M_sun 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_earth and a mass of 5.6 $\pm$ 0.7
M_earth, whereas K2-146 c has a radius of $2.59_{-0.39}^{+1.81}$ R_earth and a
mass of 7.1 $\pm$ 0.9 M_earth. 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.
