Mass fluxes J are computed for the extragalactic O stars investigated by Tramper et al .
( 2011 ; TSKK ) .
For one early-type O star , computed and observed rates agree within errors .
However , for two late-type O stars , theoretical mass-loss rates underpredict observed rates by \sim 1.6 dex , far exceeding observational errors .
A likely cause of the discrepancy is overestimated observed rates due to the neglect of wind-clumping .
A less likely but intriguing possibility is that , in observing O stars with Z / Z _ { \sun } \sim 1 / 7 , TSKK have serendipitously discovered an additional mass-loss mechanism not evident in the spectra of Galactic O stars with powerful radiation-driven winds .
Constraints on this unknown mechanism are discussed . In establishing that the discrepancies , if real , are inescapable for purely radiation-driven winds , failed searches for high- J solutions are reported and the importance of a numerical technique that can not spuriously create or destroy momentum stressed . The Z -dependences of the computed rates for Z / Z _ { \sun } \in ( 1 / 30 ,\ > 2 ) show significant departures from a single power law , and these are attributed to curve-of-growth effects in the differentially expanding reversing layers .
The best-fitting power-law exponents range from 0.68-0.97 .