Context : Swift J0243.6+6124 is a unique system .
It is the first and only ultra-luminous X-ray source in our Galaxy .
It is the first and only high-mass Be X-ray pulsar showing radio jet emission .
It was discovered during a giant X-ray outburst in October 2017 .
While there are numerous studies in the X-ray band , very little is known about the optical counterpart .

Aims : Our aim is to characterize the variability timescales in the optical and infrared bands in order to understand the nature of this intriguing system .

Methods : We performed optical spectroscopic observations to determine the spectral type .
Long-term photometric light curves together with the equivalent width of the H \alpha line were used to monitor the state of the circumstellar disk .
We used BVRI photometry to estimate the interstellar absorption and distance to the source .
Continuous photometric monitoring in the B and V bands allowed us to search for intra-night variability .

Results : The optical counterpart to Swift J0243.6+6124 is a V = 12.9 , O9.5Ve star , located at a distance of \sim 5 kpc .
The optical extinction in the direction of the source is A _ { V } = 3.6 mag .
The rotational velocity of the O-type star is 210 km s ^ { -1 } .
The long-term optical variability agrees with the growth and subsequent dissipation of the Be circumstellar disk after the giant X-ray outburst .
The optical and X-ray luminosity are strongly correlated during the outburst , suggesting a common origin .
We did not detect short-term periodic variability that could be associated with nonradial pulsations from the Be star photosphere

Conclusions : The long-term optical and infrared pattern of variability of Swift J0243.6+6124 is typical of Be/X-ray binaries .
However , the absence of nonradial pulsations is unusual and adds another peculiar trait to this unique source .