To provide a direct test of common envelope (CE) evolution which can be easily confirmed by observations, we (Sarna et al. 1995) recently modelled the change in the abundance ratio of 12C/13C on the surface of the lower mass star of a binary during the CE phase. The model is based on the fact that it is probable that the dwarf star accretes material during the CE phase. Since, during the CE phase, the dwarf secondary effectively exists within the atmosphere/envelope of the giant or supergiant primary, the accreted material has the abundances/composition of a giant/supergiant star. The 12C/13C ratio is known to decrease from approximately 90 in dwarf stars (in which the 13CO band at 2.3448 microns is barely visible) to approximately 10 in giants (in which the 13CO band at 2.3448 microns is fairly prominent). Hence, by measuring the 12C/13C ratio in post common envelope binaries (PCEBs) and comparing it to our models we would be able not only to confirm the CE theory but also to determine the amount of mass accreted during the CE phase and hence the initial mass of the dwarf component prior to the CE phase. We also propose an evolutionary scenario in which PCEBs with secondary component mass near 1.0 M⊙ start semi-detached evolution almost immediately after the CE phase. The progenitor system is a wide binary consisting of a 3 M⊙ primary with a 1.0 M⊙ secondary star. The primary evolves to fill its Roche lobe when it has a 0.6 M⊙ C–O core, with two shell burning regions. Such a star has a thick convective envelope, mass transfer is dynamically unstable and a common envelope forms. After the CE phase we are left with a close detached binary consisting of the primary’s core (0.6 M⊙) and the secondary (1.0 M⊙) main sequence star. Shortly afterwards the secondary fills its Roche lobe and mass transfer occurs (Sarna, Marks & Smith 1995). The system now evolves as a semi-detached binary (CV), transferring material to the white dwarf which undergoes nova outbursts. Figs. 1 and 2 show the isotopic ratios of 12C/13C and 16O/17O during the semi-detached evolution. In Fig. 1 the secondary did not accrete any material during CE evolution whilst in Fig. 2 the secondary accreted 0.2M⊙ during the CE stage.