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Ultra-short period (USP) planets are a class of low mass planets with periodsshorter than one day. Their origin is still unknown, with photo-evaporation ofmini-Neptunes and in-situ formation being the most credited hypotheses.Formation scenarios differ radically in the predicted composition of USPplanets, it is therefore extremely important to increase the still limitedsample of USP planets with precise and accurate mass and density measurements.We report here the characterization of an USP planet with a period of 0.28 daysaround K2-141 (EPIC 246393474), and the validation of an outer planet with aperiod of 7.7 days in a grazing transit configuration. We derived the radii ofthe planets from the K2 light curve and used high-precision radial velocitiesgathered with the HARPS-N spectrograph for mass measurements. For K2-141b wethus inferred a radius of $1.51\pm0.05~R_\oplus$ and a mass of$5.08\pm0.41~M_\oplus$, consistent with a rocky composition and lack of a thickatmosphere. K2-141c is likely a Neptune-like planet, although due to thegrazing transits and the non-detection in the RV dataset, we were not able toput a strong constraint on its density. We also report the detection ofsecondary eclipses and phase curve variations for K2-141b. The phase variationcan be modeled either by a planet with a geometric albedo of $0.30 \pm 0.06$ inthe Kepler bandpass, or by thermal emission from the surface of the planet at$\sim$3000K. Only follow-up observations at longer wavelengths will allow us todistinguish between these two scenarios.

An ultra-short period rocky super-Earth with a secondary eclipse and a Neptune-like companion around K2-141