Stellar evolution in fast forward

A few exotic stars are racing across the Hertzsprung-Russell diagram on a h uman timescale. Apparently, they are violating the stellar evolutionary spe ed limits set by physics. Are they really? The prototype object, FG Sge, co ntinues to surprise and amaze the current generation of astronomers with it s antics just as it did the previous generation 30 years ago. Now, FG Sge h as been joined by two other examples evolving even more rapidly. Here we re view the observed behaviour and theoretical speculations for three such sys tems: FG Sge, Sakurai's object, and V605 Aquilae. [GRAPHICS]