swell up well beyond their ZAMS radius, mainly due to shell deuterium burning (Palla & Stahler 1992). What is more, the higher the accretion rate the more the radius increases and extends to higher masses before the rapid contraction onto the main sequence occurs. The calculations by Palla & Stahler only consider accretion rates up to $\sim$10$^{-4}$  and masses up to 15 M  (see Figure \[fig:palla\]), but the current thinking points towards accretion rates as high as $\sim$10$^{-3}$ . Inspection of Figure \[fig:palla\] shows that we would then expect a much greater swelling, up to around 30 R, before the contraction to the main sequence at around 30 M  (L$\sim 10^{5}$ L) for these rates of accretion (Palla, private communication).

![Variation of the radius of an accreting star as it grows in mass with the steady accretion rates shown. From Palla & Stahler (1992).[]{data-label="fig:palla"}](palla.ps){width="10cm"}

The key point here is that a larger radius means lower effective temperatures. If ongoing accretion keeps the effective temperature below about 30 000 K then there is no need to invoke any mechanism to quench the H II region since there will not be a significant Lyman continuum output from the star in any case. 