Context :

Aims : This study seeks to provide empirical constraints on the different physical components that can act to yield temporal variability in predominantly or partially wind-formed optical lines of luminous OB stars , and thus potentially affect the reliable determination of fundamental parameters , including mass-loss rates via clumped winds .

Methods : Using time-series spectroscopy from epochs spread over \sim 4 years , we present a case study of the O9.5 supergiant \alpha Cam .
We demonstrate that the HeI \lambda 5876 ( 2 ^ { 3 } P ^ { 0 } –3 ^ { 3 } D ) line is an important diagnostic for photospheric and wind variability in this star .
The actions of large radial velocity shifts ( up to \sim 30 km s ^ { -1 } ) in the photospheric absorption lines can also affect the morphology of the H _ { \alpha } line profile , which is commonly used for measuring mass-loss rates in massive stars .

Results : We identify a 0.36-day period in subtle absorption profile changes in HeI \lambda 5876 , which likely betrays photospheric structure , perhaps due to low-order non-radial pulsations .
This signal persist over \sim 2 months , but it is not present 2 years later ( November 2004 ) ; it is also not seen in the stellar wind components of the line profiles .
Using a pure H _ { \alpha } line-synthesis code we interpret maximum changes in the red-ward and peak emission of \alpha Cam in terms of mass-loss rate differences in the range \sim 5.1 \times 10 ^ { -6 } to 6.5 \times 10 ^ { -6 } M _ { \odot } yr ^ { -1 } .
However , the models generally fail to reproduce the morphology of blueward ( possibly absorptive ) regions of the profiles .

Conclusions : The optical line profiles of \alpha Cam are affected by ( i ) deep-seated fluctuations close to , or at , the photosphere , ( ii ) atmospheric velocity gradients , and ( iii ) large-scale stellar wind structure .
This study provides new empirical perspectives on accurate line-synthesis modelling of stellar wind signatures in massive luminous stars .