Context :

Aims : The work is aimed at a study of the circumstellar disk of the bright classical binary Be star \pi Aqr .

Methods : We analysed variations of a double-peaked profile of the H _ { \alpha } emission line in the spectrum of \pi Aqr that was observed in many phases during \sim 40 orbital cycles in 2004–2013 .
We applied the Discrete Fourier Transform ( DFT ) method to search for periodicity in the peak intensity ( V/R ) ratio .
Doppler tomography was used to study the structure of the disk around the primary .

Results : The dominant frequency in the power spectrum of the H _ { \alpha } V/R ratio is 0.011873 day ^ { -1 } that correspond to a period of 84.2 ( 2 ) days and is in agreement with the earlier determined orbital period of the system , P _ { orb } = 84.1 days .
The V/R ratio shows a sinusoidal variation phase-locked with the orbital period .
Doppler maps of all our spectra show a non-uniform structure of the disk around the primary : a ring with the inner and outer radii at V _ { in } \approx 450 km s ^ { -1 } and V _ { out } \approx 200 km s ^ { -1 } , respectively , along with an extended stable region ( spot ) at V _ { x } \approx 225 km s ^ { -1 } and V _ { y } \approx 100 km s ^ { -1 } .
The disk radius of \approx 65 R _ { \odot } = 0.33 AU was estimated assuming Keplerian motion of a particle on a circular orbit at the disk outer edge .

Conclusions :