---
author:
- 'R. K. Prinja'
- 'D. L. Massa'
date: 'Received April, 2013; accepted 2013'
title: 'UV diagnostic of porosity-free mass-loss estimates in B stars'
---

Introduction
============

The winds of massive stars power and enrich the interstellar medium, control the evolution of the stars, determine their ultimate fate and the nature of their remnants, determine the appearance of the Hertzsprung-Russell diagram of young, massive clusters and star-bursts, and play a major role in the initial stages of massive star cluster formation and their subsequent evolution. The sensitivity of these effects to stellar winds is especially important in view of recent developments which suggest that the mass loss rates of OB stars may be in question at the [*order-of-magnitude*]{} level, and not just a few $\%$ (e.g. Massa et al.2003, Repolust et al. 2004; Bouret et al. 2005; Prinja et al. 2005; Fullerton et al. 2006; Puls et al. 2006; Cohen et al. 2006). Investigations of clumping and structures in the stellar winds are pivotal in understanding how large discrepancies between different mass loss rate determinations can arise. There are three traditional mass loss diagnostics for OB stars; all of which assume a homogeneous, spherically symmetric wind with a 