  Observational evidence of a hydrodynamically evaporating upper atmosphere of
HD209458b (Vidal-Madjar et al. 2003; 2004) and recent theoretical studies on
evaporation scenarios of ``Hot Jupiters'' in orbits around solar-like stars
with the age of the Sun indicate that the upper atmospheres of short-periodic
exoplanets experience hydrodynamic blow-off conditions resulting in loss rates
of the order of about 10^10 - 10^12 g s^-1 (Lammer et al. 2003; Yelle 2004;
Baraffe et al. 2004; Lecavlier des Etangs et al. 2004; Jaritz et al. 2005, Tian
et al. 2005; Penz et al. 2007). By studying the effect of the Roche lobe on the
atmospheric loss from short-periodic gas giants we found, that the effect of
the Roche lobe can enhance the hydrodynamic evaporation from HD209458b by about
2 and from OGLE-TR-56b by about 2.5 times. For similar exoplanets which are
closer to their host star than OGLE-TR-56b, the enhancement of the mass loss
can be even larger. Moreover, we show that the effect of the Roche lobe raises
the possibility that ``Hot Jupiters'' can reach blow-off conditions at
temperatures which are less than expected (< 10000 K) due to the stellar X-ray
and EUV (XUV) heating.
