The Monte Carlo line force as a fraction of the effective gravity in the region around the sonic point for the best-β solution of the O6 V (top panel) and O6.5 V (bottom panel) stars. The contribution of iron is shown separately (blue dotted line). Note that in the case of the O6.5 star the line acceleration does not balance the effective gravity at the sonic point. This is interpreted as a failure to support a line driven wind.