New constraints on the planetary system around the young active star AU Mic

AU Mic is a young, active star whose transiting planet was recently detected. We report our analysis of its TESS data, where we modeled the BY Draconis type quasi-periodic rotational modulation by starspots simultaneously to flaring activity and planetary transits. measured flare occurrence rate 6.35 flares per day for with amplitudes in range $0.06\% < f_{\rm max} 1.5\%$ flux. employed Bayesian MCMC model five transits b, improving constraints on parameters. The radius $4.07\pm0.17$~R$_{\oplus}$ mean density $1.4\pm0.4$~g~cm$^{-3}$ confirms that it Neptune-size moderately inflated planet. While single feature possibly due second previously reported former detection two additional transit-like events new observations July 2020. This represents substantial evidence (AU c) system. analyzed three obtained an orbital period $18.859019\pm0.000016$~d $3.24\pm0.16$~R$_{\oplus}$, which defines as warm expected mass 2.2~M$_{\oplus}$~$< M_{\rm c} $25.0~M$_{\oplus}$. planets system are near 9:4 mean-motion resonance. show this configuration dynamically stable should produce transit-timing variations (TTV). Our non-detection significant TTV b suggests upper limit c $<7$~M$_{\oplus}$, indicating also likely be inflated. As young multi-planet at least planets, becomes key study atmospheres infant planet-planet planet-disk dynamics early stages evolution.