Constraining the interior of exoplanets is a very degenerated problem with many solutions. An very promising possibility is the spectroscopic studuy of planets for which the atmosphere is chemically exhange with the molten planet's mantle : these are planets with a magma ocean. Rocky exoplanets with a magma ocean at their surfacea are those with equilibrium temperature temperature > 1400K. They are a new category of planets that still need to be characterized and the unambiguous detection of a magma ocean is still elusive. I will discuss about the the atmosphere of magma ocean exoplanets, and in particular the effect of Hydrogen and volatiles (water, cO2) and show spectral signatures when a magma ocean is in contact with the atmosphere. I will extend the discussion to sub-neptunes, with equilibrium temperatures < 1000K , and show that they can have a magma ocean hidden under a thick atmospheric layer that may be an explantion to the high metallicities observed in some of sub-neptunes. I will discuss possibiilities to constrains magma-oceans compositios on sub-neptunes.