TOI-836: A super-Earth and mini-Neptune transiting a nearby K-dwarf

We present the discovery of two exoplanets transiting TOI-836 (TIC 440887364) using data from TESS Sector 11 and Sector 38. TOI-836 is a bright ($T=8.5$ mag), high proper motion ($\sim 200$ mas yr${}^{-1}$), low metallicity ([Fe/H]$\approx -0.28$) K-dwarf with a mass of $0.68\pm 0.05$ M${}_{\odot }$ and a radius of $0.67\pm 0.01$ R${}_{\odot }$. We obtain photometric follow-up observations with a variety of facilities, and we use these data-sets to determine that the inner planet, TOI-836 b, is a $1.70\pm 0.07$ R${}_{\oplus }$ super-Earth in a 3.82 day orbit, placing it directly within the so-called ‘radius valley’. The outer planet, TOI-836 c, is a $2.59\pm 0.09$ R${}_{\oplus }$ mini-Neptune in an 8.60 day orbit. Radial velocity measurements reveal that TOI-836 b has a mass of $4.5\pm 0.9$ M${}_{\oplus }$ , while TOI-836 c has a mass of $9.6\pm 2.6$ M${}_{\oplus }$. Photometric observations show Transit Timing Variations (TTVs) on the order of 20 minutes for TOI-836 c, although there are no detectable TTVs for TOI-836 b. The TTVs of planet TOI-836 c may be caused by an undetected exterior planet.