Large components of principal series and characteristic cycles

For a semisimple quasi-split real linear group which is also maximal in Kos tant's sense, a theorem of Vogan asserts that there is a unique composition factor that is large in any principal series. We give a proof of this theo rem using results of Schmid and Vilonen that establish a conjecture of Barb asch and Vogan about characteristic cycles. As a byproduct, we obtain some information about the characteristic cycles of the localized K-equivariant sheaves of these principal series.