A spectroscopic study was carried out on the surface chemical abundances of CNO and several heavier elements in the primary and secondary components of 5 eclipsing close binaries around A-type ( AR Aur , \beta Aur , YZ Cas , WW Aur , and RR Lyn ) , in order to investigate the nature of chemical differences between both components ( being comparatively slow rotators alike due to tidal synchronization ) .
Regarding the systems comprising similar components , \beta Aur and WW Aur were confirmed to exhibit no compositional difference between the primary and secondary both showing almost the same Am anomaly , though the chemical peculiarities seen in the component stars of AR Aur show distinct differences ( HgMn star and Am star ) .
In contrast , as to the systems ( YZ Cas and RR Lyn ) consisting of considerably different ( A and early-F ) components , the surface abundances are markedly different between the primary ( Am ) and secondary ( normal ) .
These observational results may indicate T _ { eff } -dependent characteristics regarding the chemical anomalies of non-magnetic stars on the upper main sequence : ( 1 ) In the effective temperature range of 10000 { K } \ga T _ { eff } \ga 7000 { K } , rotational velocity is the most important factor for determining the extent of Am peculiarity .
( 2 ) However , the emergence of Am phenomenon seems to have a lower T _ { eff } limit at \sim 7000 K , below which no abundance anomaly is observed regardless of stellar rotation .
( 3 ) The transition from Am anomaly ( mild deficiency in CNO ) to HgMn anomaly ( unusually large N depletion ) is likely to take place as T _ { eff } increases from \sim 10000 K to \sim 11000 K .