**Table 1:** Relevant stellar parameters for Cyg OB2 No. 9 adopted in this paper. The numbers are taken from Bieging et al. (1989, BAC), Leitherer (1988, L) and Herrero et al. (1999, HCVM).
Parameter		Ref.
$v_{\rm rot} \sin i~({\rm km~s^{-1}})^a$	145	BAC
$T_{\rm eff}~({\rm K})^b$	44 500	HCVM
$R_*~({R_{\odot}})$	22	HCVM
$d~({\rm kpc})$	1.82	BAC
$v_\infty~({\rm km~s^{-1}})$	2 900	L
$\dot{M}~({M_{\odot}\rm ~yr^{-1}})$	$2.0\times 10^{-5}$	L
$L_*~({L_{\odot}})$	$1.74\times 10^6$	HCVM

^a We follow White (1985) in assuming that $v_{\rm rot}= 250~{\rm km~s^{-1}}$ , which is not in contradiction with the value for $v_{\rm rot} \sin i$ .
^b The wind temperature is assumed to be 0.3 times the stellar effective temperature (Drew 1989).

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	1 Introduction
	2 Emission layers
	3 Application and results
	4 Discussion and conclusions
	References