Recently, Neuhäuser et al. (2005) discovered a sub-stellar companion around the classical T 
Tauri star GQ Lup by direct detection, orbiting at a distance of ~ 100 AU.
Further planet candidates at closer distances are searched for by the radial velocity 
technique. Unfortunately, classical T Tauri stars can exhibit strong spot activity. This can 
mimic radial velocity signals of an orbiting planet. 
                     
We have monitored GQ Lup A photometrically in two epochs, took archival data into account 
and  could thus determine a periodicity of the brightness variations. 
By comparing the photometric amplitude in different wavelength bands we were able to ascribe 
the variations to surface activity of the star and could therefore determine the rotational 
period of 8.45 +- 0.2 days.
                       
Assuming luminosity, temperature and the newly found period for this K7V star, the 
equatorial rotational velocity is about 15.2 km/s. With the v sini = 6.8 +- 0.4 km/s  from 
Guenther et al. (2005) this implies an inclination of GQ Lup A of about 27 +- 5° and thus an 
almost pole-on view onto the star. This result may very likely also be the inclination of 
the disk around GQ Lup A, and possibly also the inclination of its companion.