For the hot exoplanets CoRoT-24b and CoRoT-24c , observations have provided transit radii R _ { T } of 3.7 \pm 0.4 R _ { \oplus } and 4.9 \pm 0.5 R _ { \oplus } , and masses of \leq 5.7 M _ { \oplus } and 28 \pm 11 M _ { \oplus } , respectively .
We study their upper atmosphere structure and escape applying an hydrodynamic model .
Assuming R _ { T } \approx R _ { PL } , where R _ { PL } is the planetary radius at the pressure of 100 mbar , we obtained for CoRoT-24b unrealistically high thermally-driven hydrodynamic escape rates .
This is due to the planet ’ s high temperature and low gravity , independent of the stellar EUV flux .
Such high escape rates could last only for < 100 Myr , while R _ { PL } shrinks till the escape rate becomes less than or equal to the maximum possible EUV-driven escape rate .
For CoRoT-24b , R _ { PL } must be therefore located at \approx 1.9 - 2.2 R _ { \oplus } and high altitude hazes/clouds possibly extinct the light at R _ { T } .
Our analysis constraints also the planet ’ s mass to be 5 - 5.7 M _ { \oplus } .
For CoRoT-24c , R _ { PL } and R _ { T } lie too close together to be distinguished in the same way .
Similar differences between R _ { PL } and R _ { T } may be present also for other hot , low-density sub-Neptunes .