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We report on the precise radial velocity follow-up of TOI-544 (HD 290498), abright K star (V=10.8), which hosts a small transiting planet recentlydiscovered by the Transiting Exoplanet Survey Satellite (TESS). We collected122 high-resolution HARPS and HARPS-N spectra to spectroscopically confirm thetransiting planet and measure its mass. The nearly 3-year baseline of ourfollow-up allowed us to unveil the presence of an additional, non-transiting,longer-period companion planet. We derived a radius and mass for the innerplanet, TOI-544b, of 2.018 $\pm$ 0.076 R$_{\oplus}$ and 2.89 $\pm$ 0.48M$_{\oplus}$ respectively, which gives a bulk density of $1.93^{+0.30}_{-0.25}$g cm$^{-3}$. TOI-544c has a minimum mass of 21.5 $\pm$ 2.0 M$_{\oplus}$ andorbital period of 50.1 $\pm$ 0.2 days. The low density of planet-b implies thatit has either an Earth-like rocky core with a hydrogen atmosphere, or acomposition which harbours a significant fraction of water. The compositioninterpretation is degenerate depending on the specific choice of planetinterior models used. Additionally, TOI-544b has an orbital period of 1.55 daysand equilibrium temperature of 999 $\pm$ 14 K, placing it within the predictedlocation of the radius valley, where few planets are expected. TOI-544b is atop target for future atmospheric observations, for example with JWST, whichwould enable better constraints of the planet composition.

TOI-544 b: a potential water-world inside the radius valley in a two-planet system