clapp oscillator

Hi friends,
I am reading oscillator design documents that introduces the colpitts and clapp oscillator.In the text,it introduces clapp oscillator can reduce frequency drift adding the cap serial with inductor compared with colpitts oscillator.I can not get the reason.Anyone kindly give me interpretion?
Thanks

Clapp oscillator actually is a modified Colpitts (using a series capacitor in the tuned LC circuit). Because the Clapp series coupling capacitor has lower value than the Colpitts coupling capacitor, there will be less influence to the active element (and feedback capacitors) into the LC tank circuit, this creating a better frequency stability.

hi..i am quite new in this topic.. what is your reference(book, article) to say that?
thanks

From "RF and Microwave Transsitor Oscillator Design" by ANDREI GREBENNIKOV