About 150 solar masses, they speculate:
http://arxiv.org/abs/astro-ph/0310860
The observed masses of the most massive stars do not surpass about 150Msun. This may either be a fundamental upper mass limit which is defined by the physics of massive stars and/or their formation, or it may simply reflect the increasing sparsity of such very massive stars so that observing even higher-mass stars becomes unlikely in the Galaxy and the Magellanic Clouds. It is shown here that if the stellar initial mass function (IMF) is a power-law with a Salpeter exponent (alpha=2.35) for massive stars then the richest very young cluster R136 seen in the Large Magellanic Cloud (LMC) should contain stars with masses larger than 750Msun. If, however, the IMF is formulated by consistently incorporating a fundamental upper mass limit then the observed upper mass limit is arrived at readily even if the IMF is invariant. An explicit turn-down or cutoff of the IMF near 150Msun is not required; our formulation of the problem contains this implicitly. We are therefore led to conclude that a fundamental maximum stellar mass near 150Msun exists, unless the true IMF has alpha>2.8.
Sooooooo . . . THANK YOU VERY MUCH, mvpel!! Very interesting. I knew there was some kind of limit. :-)
Wow, a 750+MSun star would be an awesome thing, if such exists. No doubt about that! (Wouldn't want to be ANYWHERE near it when it went supernova either!)