The existence of pulsations in HgMn stars is still being debated .
To provide the first unambiguous observational detection of pulsations in this class of chemically peculiar objects , the bright star HD 45975 was monitored for nearly two months by the CoRoT satellite .
Independent analyses of the light curve provides evidence of monoperiodic variations with a frequency of 0.7572 d ^ { -1 } and a peak-to-peak amplitude of \sim 2800 ppm .
Multisite , ground-based spectroscopic observations overlapping the CoRoT observations show the star to be a long-period , single-lined binary .
Furthermore , with the notable exception of mercury , they reveal the same periodicity as in photometry in the line moments of chemical species exhibiting strong overabundances ( e.g. , Mn and Y ) .
In contrast , lines of other elements do not show significant variations .
As found in other HgMn stars , the pattern of variability consists in an absorption bump moving redwards across the line profiles .
We argue that the photometric and spectroscopic changes are more consistent with an interpretation in terms of rotational modulation of spots at the stellar surface .
In this framework , the existence of pulsations producing photometric variations above the \sim 50 ppm level is unlikely in HD 45975 .
This provides strong constraints on the excitation/damping of pulsation modes in this HgMn star .