The chemical composition of Algol systems- II. The carbon and nitrogen abundances of the secondaries of U CEP and U Sge.

A spectrum synthesis analysis of Digicon spectra of CH and CN in the secondaries of U Cep and U Sge shows that they are carbon deficient, [C/Fe] ∼ -0.5, and nitrogen rich, [N/Fe] ∼ +0.5. The metal abundance of both secondaries is normal, [Fe/H] = 0.0 ± 0.3.

The C deficiency and N overabundance of the secondaries are the result of conversion of C to N by the CN cycle, while the secondaries were on the main sequence, followed by mixing to the surface. These abundances are therefore observational evidence in favour of the generally accepted idea that the secondaries of semi- detached systems are post- main- sequence objects.

The C deficiency and N overabundance of the secondaries are more marked than in the typical single G or K giant. We interpret this as a result of mass loss of the unprocessed surface material prior to the onset of mixing. At the time of mixing in the secondaries there was less unprocessed material and therefore less dilution of the CN cycle processed material than in a single G or K giant.

Mass loss subsequent to mixing does not alter the surface composition of the secondaries. This explains why secondaries that have suffered much more extensive mass loss than those of U Cep or U Sge do not show the extremes of composition that one would expect in the absence of mixing. An example is the secondary of S Cnc (K2), which has a mass of only 0.2M_sun and for which - from a preliminary analysis of CH - we find [C/Fe] ∼ -0.5.