Most stars evolve from the main sequence to become red giants. As red giants, their atmospheres are generally rich in hydrogen. ... During this process, they lose nearly all their outermost layers. Apparently, some lose all of their hydrogen, revealing layers of helium and carbon. These are the hydrogen-deficient stars.. ... extreme helium stars, . ... The question is: How did the hydrogen-deficient stars lose their hydrogen ? ... V652 Her . ... Simon Jeffery ( csj@star.arm.ac.uk ) . ...