  We present high-resolution (R~60,000) optical spectra of a carefully selected
sample of heavily obscured and presumably massive O-rich Asymptotic Giant
Branch (AGB) stars in the Magellanic Clouds (MCs). We report the discovery of
strong Rb I lines at 7800 A in four Rb-rich LMC stars at luminosities equal to
or greater than the standard adopted luminosity limit for AGB stars
(Mbol~-7.1), confirming that "Hot Bottom Burning" (HBB) may produce a flux
excess in the more massive AGB stars. In the SMC sample, just one of the five
stars with Mbol<-7.1 was detected in Rb; the other stars may be massive red
supergiants. The Rb-rich LMC AGB stars might have stellar masses of at least
~6-7 Msun. Our abundance analysis show that these Rb-rich stars are extremely
enriched in Rb by up to 10^3-10^5 times solar but seem to have only mild Zr
enhancements. The high Rb/Zr ratios, if real, represent a severe problem for
the s-process, even if the 22Ne source is operational as expected for massive
AGB stars; it is not possible to synthesize copious amounts of Rb without also
overproducing Zr. The solution to the problem may lie with an incomplete
present understanding of the atmospheres of luminous AGB stars.
