(Left panel): Hertzsprung–Russel diagram of field M-type stars with different metallicities. GJ7 (TOI-198) is depicted by the star symbol. Solar-metallicity stars come from Schweitzer et al. (2019) and low-metallicity stars were collected from Kesseli et al. (2019). Stellar metallicity is color-coded. The 5 Gyr isochrones of the BT-Settl models ([Fe/H] between solar and –2 dex produced by Allard et al. (2013) are depicted by the solid lines. (Right panel): stellar radius versus metallicity. Tracks of constant mass (Allard et al. 2013) are shown by the blue lines. Stellar T_eff is color-coded. The stellar radius axis is on a logarithmic scale.