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GJ 367 is a bright (V $\approx$ 10.2) M1 V star that has been recently foundto host a transiting ultra-short period sub-Earth on a 7.7 hr orbit. With theaim of improving the planetary mass and radius and unveiling the innerarchitecture of the system, we performed an intensive radial velocity follow-upcampaign with the HARPS spectrograph -- collecting 371 high-precisionmeasurements over a baseline of nearly 3 years -- and combined our Dopplermeasurements with new TESS observations from sectors 35 and 36. We found thatGJ 367 b has a mass of $M_\mathrm{b}$ = 0.633 $\pm$ 0.050 M$_{\oplus}$ and aradius of $R_\mathrm{b}$ = 0.699 $\pm$ 0.024 R$_{\oplus}$, corresponding toprecisions of 8% and 3.4%, respectively. This implies a planetary bulk densityof $\rho_\mathrm{b}$ = 10.2 $\pm$ 1.3 g cm$^{-3}$, i.e., 85% higher thanEarth's density. We revealed the presence of two additional non transitinglow-mass companions with orbital periods of $\sim$11.5 and 34 days and minimummasses of $M_\mathrm{c}\sin{i_\mathrm{c}}$ = 4.13 $\pm$ 0.36 M$_{\oplus}$ and$M_\mathrm{d}\sin{i_\mathrm{d}}$ = 6.03 $\pm$ 0.49 M$_{\oplus}$, respectively,which lie close to the 3:1 mean motion commensurability. GJ 367 b joins thesmall class of high-density planets, namely the class of super-Mercuries, beingthe densest ultra-short period small planet known to date. Thanks to ourprecise mass and radius estimates, we explored the potential internalcomposition and structure of GJ 367 b, and found that it is expected to have aniron core with a mass fraction of 0.91$^{+0.07}_{-0.23}$. How this iron core isformed and how such a high density is reached is still not clear, and wediscuss the possible pathways of formation of such a small ultra-dense planet.

Company for the ultra-high density, ultra-short period sub-Earth GJ 367 b: discovery of two additional low-mass planets at 11.5 and 34 days