Asteroseismic probing of internal rotation in hot B subdwarf stars: Testing spin-orbit synchronism in two close binary systems

We present internal rotation profiles derived from asteroseismology for the hot pulsating B subdwarf stars PG 1336-018 and Feige 48. These two pulsators are primaries of close binary systems of known orbital period and, therefore, provide laboratories to test, for the first time, spin-orbit synchronization as a function of depth. We show that PG 1336-018 and Feige 48 clearly rotate as solid bodies with periods equal to their orbital periods from the surface down to at least ~ 0.5 and ~ 0.3 their radius, respectively. Deep tidal locking has therefore developed within the relatively short lifetime of these stars (~ 10^8 yr).